All West Germanic (and some Romance)1 languages have “conditional” uses of certain modal verbs, in which the modal verb does not express a modal (root or epistemic) meaning, but acts more like a conditionality marker (Nieuwint 1989; van der Auwera & Plungian 1998; Nuyts et al. 2005; Boogaart 2007; Haegeman 2010b; Van Den Nest 2010). In German, this modal verb is sollen, or rather, its past (subjunctive) form sollte.

Interestingly, while V1-conditionals cannot be substituted for a wenn ‘if’-conditional in all syntactic and semantic contexts, V1-conditionals with sollte ‘should’ seem to be much less sensitive to this distinction (e.g. Reis & Wöllstein 2010). They are allowed in contexts where V1-conditionals with other verbs are excluded, such as relevance conditionals like (1) (Reis & Wöllstein 2010: 137), which Reis & Wöllstein attribute to the fact that the idiosyncratic meaning of “conditional” sollte, highlighting the hypothetical character of the antecedent proposition (Welke 1965: 99; Glas 1984: 86) and focussing its potential character, serves as overt marking of conditionality, and can therefore override the restrictions of V1-protases.

Sollte is not restricted to V1-protases, but also occurs in combination with wenn ‘if’ (or falls ‘in case’), with seemingly no or little difference in distribution. Reis & Wöllstein (2010: 137) call German sollte-V1-conditionals a Sonderfall (‘special case’) and state that

They add in a footnote

Synchronically, Breitbarth et al. (2016) give such a comparison between the distribution and the different degrees of grammaticalization of English should and German sollte, together with (Netherlands and Standard) Dutch mocht and (Belgian Colloquial) Dutch moest, while Breitbarth (2015) proposed a formal account for the diachronic development of conditional should in English. The question of how German sollte, originally a modal verb expressing obligation, could come to express a potential or conditional meaning, and how sollte-conditionals acquired their special distribution, has not yet been discussed in detail in the literature. Building on Breitbarth (2015) and Breitbarth et al. (2016), the current paper therefore sets out to describe the diachronic development of conditional sollte, and analyses it in a framework using insights from Roberts & Roussou’s (2003) Minimalist approach to grammaticalization in a cartographic analysis of modality and conditio-nals based on Cinque (1999) and Haegeman (2010a).

The diachronic case study presented in this paper is based on a Middle High German (MHG; 1050–1350) and a New High German (NHG) corpus. The MHG data come (a) from the Middle High German Conceptual Database (MHDBDB; 185,632 tokens in 595 texts)4 and (b) from the newly available Referenzkorpus Mittelhochdeutsch (ReM; 2.5 million tokens in 395 texts),5 Klein & Dipper (2016). The NHG data are drawn from the written archive of the German Reference Corpus (DeReKo; ca. 7.3 billion words in total),6 which contains texts from Germany, Austria and Switzerland, and were subdivided into two periods, (i) historical texts from 1741–1915 (older NHG) and (ii) selected sources from the archive of written language between 1985–2012 (Present-Day German, PDG).

Any conditional clauses with the modal verb suln/sollen were extracted, both syndetic and asyndetic, regardless of the modal meaning of the verb. The results of these queries were manually checked, and false hits (i.e. cases that were not conditional clauses with suln/sollen) removed. In the end, 428 clauses were obtained for MHG (362 from the MHDBDB and 66 from ReM). From the older NHG and the PDG subcorpora of the DeReKo, a random first selection of 200 clauses each was made, which after deselection of false hits resulted in 186 and 195 clauses, respectively. These were coded for (i) the meaning of the modal verb (e.g. circumstantial, or conditional), (ii) whether or not they are syndetic, (iii) the semantic linkage level (content/event, inferential or illocutionary), (iv) the degree of probability for content-level protases, (v) the tense of the verb (suln/sollen) in the protasis,7 and (vi) the tense of the verb in the apodosis.

Section 2 presents the results of the diachronic corpus study. Section 2.1 reports on the diachronic changes in the distribution of the modal meanings of suln/sollen in conditional protases, 2.2 shows how the semantic integration of sollte-conditionals developed over time, 2.3 looks at how sollte became increasingly restricted to the past (subjunctive) form, regardless of the degree of probability expressed by the conditional, or the tense of the verb in the apodosis, and 2.4 briefly turns to the competition between mugen and suln as conditional modals in MLG. Section 3 proposes an analysis of the syntax and semantics of conditional sollte. The diachronic development of conditional sollte is given a formal account in Section 4, and Section 5 concludes.

As variously observed in the literature (Welke 1965; Glas 1984; Reis & Wöllstein 2010; Breitbarth et al. 2016), sollte mostly seems to be an expression of hypotheticality or potentiality in conditional protases. The data in the corpus were coded for two main uses, modal (modifying the lexical verb), and conditional (‘in case’). Any use in which a form of suln/sollen could be interpreted as e.g. deontic or circumstantial (‘X has to/is supposed to/expected to happen, X is necessary/possible’) or buletic (‘it is/would be preferable for X to/Y wants X to happen’) were coded as modal. (2) illustrates this with examples from the three periods.

The conditional meaning (roughly, ‘in case’) is illustrated for the three periods in (3).

The conditional meaning is already prominently present in MHG, as Table 1 shows. In PDG, it is the dominant meaning. Throughout all three stages, the conditional meaning is c. 20% more frequently attested in asyndetic protases than in syndetic ones.

Also the tense and mood of suln/sollen interact differently with the conditional meaning in conditional protases: present indicative becomes increasingly restricted to the modal meaning(s), while past (subjunctive) becomes more tightly associated with the conditional meaning. In MHG, and very marginally still in older NHG, the conditional meaning could also be expressed by suln/sollen in the present indicative, (4).8 In PDG, while the conditional meaning can only be expressed using the past (subjunctive) form sollte(n), the modal meaning is still expressed by the past (subjunctive) in about a quarter of all cases, illustrated by (5).

In the following subsections, we restrict the discussion to suln/sollen in the conditional meaning. We return to the analysis of the conditional meaning of sollte and its diachronic development from modal to conditional in Sections 3 and 4.

The semantic relationship between the protasis and the apodosis in a conditional may be situated on (i) the content (or predictive) level, (ii) the epistemic (or inferential) level, or (iii) the speech-act (or illocutionary) level (Sweetser 1990; Dancygier & Sweetser 2005). In a predictive conditional, the content of the proposition in the protasis forms the prerequisite for the truth of the apodosis proposition in the real world, (6). In an epistemic conditional, the protasis forms the epistemic basis for an inference as to the truth of the apodosis proposition, (7). In a speech-act (or relevance) conditional, the protasis expresses a circumstance under which the apodosis provides relevant information, (8).

Content-level linkage is the most frequent type in all three subcorpora (Table 3), but there are clear differences between them. In MHG, nearly a quarter of all conditionals with conditional suln are epistemically linked, while in older NHG, the share of conditionals linked at the speech-act level is much larger with 23%. PDG has the highest share of predictive conditionals, ca. 25% more than in MHG and older NHG.

Predictive conditionals can express three degrees of probability: realis, potentialis, and irrealis/counterfactual (Quirk et al. 1985; Leech 2004). The distinction is normally marked by specific tense/mood patterns (consecutio temporum/sequence of tenses), illustrated for suln/sollen-conditionals with examples from the corpus in (9a)–(9c). As variously noted in the literature on German and English (Welke 1965; Van Den Nest 2010: 117–118, 123; Breitbarth 2015), protases with conditional sollte and should often diverge from these patterns, leading to a tense mismatch between protasis and apodosis. (9d) is a realis conditional, but the tense of sollen is formally past (subjunctive).

Table 4 shows that MHG still adhered to the classical consecutio temporum. In NHG, counterfactual conditionals are no longer possible with conditional sollen, which is probably due to its meaning, which Reis & Wöllstein (2010: 137) circumscribe as hypothetical, but focussing the potential realization of the event, hence excluding counterfactual construal. While older NHG still has a small share of conditional sollen in present tense, (10), leading to pattern A (10) (see also Section 2.1), the only possibilities in PDG are pattern B (9b), or the mixed pattern (9d).

The share of tense mismatches has increased from c. 13% in MHG to nearly 62% in older NHG, and to nearly 70% in PDG, making it the neutral pattern in the language today.

As observed by Van Den Nest (2010) and Breitbarth et al. (2016: 302), suln is not the only modal verb able to express a conditional meaning in MHG, mugen ‘may’ is attested with this meaning as well,9 besides a number of modal meanings, (11), which show a certain correlation with the position of the finite verb in the protasis, but are altogether more frequent than the conditional meaning, as Table 5 shows.

Like conditional suln in MHG, conditional mugen still seems to obey the rules of the consecutio temporum, (12). Tense mismatches between protasis and apodosis are not attested in the corpora used.

In older NHG and PDG, conditional uses of mögen ‘may’ are no longer attested.

The diachronic corpus study has shown that conditional protases with suln/sollen increasingly occur in predictive conditionals, and increasingly with (formally) past (subjunctive) of the modal (sollte), regardless of the degree of probability expressed. The initial competition for the conditional meaning between MHG suln and another modal verb, mugen ‘may’, was early on overcome, and is no longer attested in the older NHG and PDG corpora. It can be concluded that from MHG to PDG, suln > sollte becomes increasingly restricted in its paradigmatic and syntagmatic variability, which following e.g. Lehmann’s (1995) parameters points to a higher degree of grammaticalization. In what follows, we first look at the syntax and semantics of conditional sollte from a synchronic point of view before proposing an account for its grammaticalization that led to the current state of affairs.

I propose that the conditional use of sollte developed out of lower modal meanings by upward reanalysis (Roberts & Roussou 2003) through the functional hierarchy. The question now is in which steps this development proceded. In the literature, three options are sketched: First, according to van der Auwera & Plungian (1998), who discuss the grammaticalization paths of modal meanings more generally, “postmodal” meanings such as conditional develop out of epistemic necessity or possibility. Second, for the development of sollte more specifically, Van Den Nest (2010) surmises a deontic input meaning. Finally, according to Fritz (1997a), the conditional meaning of sollte arises from a combination of the futurity sollen could express in older stages of the language up to ENHG and the uncertainty of the past subjunctive.12

The problem with a development from epistemic necessity or possibility to conditional as envisaged by van der Auwera & Plungian is the unavailability of epistemic modals in conditional protases. Also in terms of Cinque’s hierarchy, epistemic modality, expressing a speaker’s belief about a proposition, is hierarchically higher than the modification of a proposition as achieved by expressions of modality.

In favour of the hypothesis that the temporal (future) use of suln was the input to the grammaticalization of the conditional use, suln was the most frequent periphrastic future marker in MHG (Diewald 1999: 321), and was only in the 14^th through 16^th centuries replaced by the periphrastic expression with werden (Ebert et al. 1993: 391f). A problem with assuming upwards reanalysis from Tense_future to Mood_irrealis is that Mood_irrealis is below Tense_future in Cinque’s hierarchy. While it is possible that the hierarchy needs a revision,13 there are two arguments that make a development out of a future use less likely. First, the future meaning of suln was never fully developed. As argued by Zeman (2013), the temporal and modal meanings of suln in MHG were highly context-dependent, making it difficult to speak of a clearly grammaticalised future periphrasis, which could be captured as merger of suln in Tense_future under Roberts & Roussou’s analysis.14 Given the persistence of modal overtones even in the future use of suln (cf. also Diewald & Habermann 2005), it is more likely that suln was originally merged low, e.g. in Mod_volitional (because of its bouletic meaning) or perhaps Mod_obligation, and moved to Tense_future (in temporal use) or Mood_irrealis (in conditional use). Second, the conditional use of suln is already very well established in MHG. Furthermore, sculan already had a special role in conditionals in OHG. In Notker’s Boethius-translation, the present indicative (though not the past subjunctive) of sol ‘shall’ is used always against the Latin original, and replaces the Latin potential mood (∼ ‘in case …’), while other modal verbs (mag, uuile ‘may, want’) used in conditionals largely follow the Latin original (Furrer 1971: 56–7). We can take this to indicate that the conditional use of suln/sollen is quite old, and had been established/grammaticalized earlier that the future use, though both may have arisen at similar times.

There is a fourth possibility that has not yet been discussed in the literature. Given the early availability of OHG sculan as translation of Latin potentialis in conditionals, and given the common bouletic use of sollte,15 an intermediate step in the reanalysis from root modality to conditional (deontic, as surmised by Van Den Nest) may be volitional modality and alethic possibility, which lie between them on Cinque’s (1999) hierarchy, (20).

According to von Fintel (2006: 21–22), “bouletic modality […] concerns what is possible or necessary given a person’s desires”; “it has a circumstantial modal base and an ordering source based on a relevant person’s desires”. This would justify locating it in Cinque’s Mod_volitional. The semantics associated with the conditional protasis as a whole could then give rise to the interpretation of solte as expressing (alethic) possibility, the meaning also seen in (14c). Both bouletic (21a) and (alethic) possibility uses (21b) are in fact attested in MHG.

An additional argument in favour of the assumption that conditional sollte arose via Mod_volitional and Mod_{(alethic) possibility} is the fact that conditional suln in MHG competed in its conditional use with mügen ‘may’, as shown in Section 2.4 above. Like MHG suln, mugen had (and still has) volitional and possibility uses as well, as seen in (11) in Section 2.4 above.

That is, conditional suln/sollen started out as an exponent of Mood_volitional and Mod_{(alethic) possibility} in conditional protases, and over time came to merge directly in Mood_irrealis, after the earlier movement from these heads was lost due to upwards reanalysis.16 The fact that MHG conditionally used suln still largely obeys the sequence of tenses and allows present tense in conditional protases (cf. Table 4) indicates that it was still merged in a modal projection, and that upward reanalysis had not yet taken place. That the conditional meaning is not always easy to disentangle from a “modal” meaning, cf. (22a) vs. (22b), is expected under the current analysis: “modal” meanings are possible still today, and under Roberts & Roussou’s (2003) approach, the preservation of the old meaning is a case of a lexical split, or divergence characteristic of grammaticalization.

While offering an account for the rise of the conditional use of suln/sollte, this proposed upwards reanalysis in (20) still fails to account for the diachronic changes in the distribution of conditional sollte and of sollte-protases, viz. the increasing restriction to past subjunctive, the rise of the “mixed” tense-mood pattern, the increasing restriction to content-level conditionals, and for the special behaviour of sollte-V1-conditionals compared to other V1-conditionals. Such an account will be attempted in the next Subsection.

As noted above, the connectedness with contextual factors, especially for older stages of the language, makes the exact analysis of modal meaning difficult (e.g. Zeman 2013). Note, however, that the conditional meaning is already attested in MHG, and very frequently so, too. As far as a distinction between circumstantial and conditional uses is possible (cf. ambiguities such as exemplified in (22a)), there is a striking correlation in the corpus used for the present study, which is diachronically becoming more pronounced, between the tense and mood of the modal and its interpretation: the circumstantial use becomes restricted to present tense, while the conditional use becomes restricted to the past tense (see Table 2 in Section 2.1).

That is, the past subjunctive is increasingly associated with the hypothetical conditional, irrealis mood, meaning of conditional sollen. In asyndetic conditionals, sollte can now only have the conditional meaning, while in syndetic ones, where there is a conditional complementizer wenn ‘if’, it is still free to be interpreted as deontic. This observation points at a syntagmatic fixation of conditional sollte that is expected if it is undergoing a grammaticalization process (Lehmann 1995).

Besides the loss of competing modals expressing the conditional meaning, viz. MHG mugen, and hence of paradigmatic variability, there are other syntagmatic restrictions building up, too, and hence, there is further grammaticalization according to Lehmann’s (1995) parameters. As seen in Section 3, example (17b), conditional sollte can only be finite. This finiteness restriction points at it being interpreted above Tense (Erb 2001). The fact that it, like epistemic modals, does not express temporal information is also witnessed by its (historically increasingly) frequent combination with present tense apodoses (cf. Table 4), despite it invariably being past subjunctive in PDG (Table 2) suggests that it is interpreted above Tense_past. Given the (partial) hierarchy in (15), this indicates that conditional sollte is in the process of a further upward reanalysis.

Recall that under Haegeman’s (2010b) analysis, the conditional modal verb in Mood_irrealis fronts to left periphery together with conditional operator in verb-initial conditionals, (23a). Reis & Wöllstein’s (2010: 137) intuition that sollte in V1-conditionals seems to be functionally equivalent to a conditional complementizer like falls can now be interpreted as conditional sollte being in the process of becoming an exponent of conditional C by upward reanalysis, (23b). As shown by Van Den Nest (2010), verb-first clauses could already early on in the history of German be used as conditional protases. Therefore, the development of sollte as a pure conditional marker could piggy-back off this generally available option.17

Such a(n ongoing) reanalysis can at once account for the loss of temporal distinctions in conditional protases with sollte. In particular, conditional sollte is no longer associated with past tense (despite being morphologically past subjunctive), but just expresses conditionality. This accounts for the increased frequency of predictive conditionals in PDG compared to MHG. This sets it apart from the deontic modal which still enjoys full paradigmatic variability, demonstrating the divergence, or lexical split characteristic of grammaticalization (Hopper 1991: 24f). It can also help account for the higher incidence of conditional sollte in V1-protases, which in PDG is ca. 23% higher than in syndetic protases (see Table 1).

There are a number of potential counterarguments to conditional sollte being on its way to becoming a C-element.18 First, if sollte were a complementizer like falls ‘in case’, one would expect (24b) to be equally acceptable as (24a). However, the difference in acceptability with a V1-conditional without sollte (24c) is suggestive of the contribution of sollte. Generally, V1-protases cannot follow the matrix clause (Reis & Wöllstein 2010). The fact that a sollte-V1-protasis is more acceptable in this position therefore seems to suggest that there is in fact a difference between sollte-V1-protases and V1-protases without sollte.19

Second, one might object that conditional sollte has the same interpretation in conditional protases without movement of sollte to C. However, I only claim here that sollte is in the process of becoming a conditional marker, with the potential of later becoming a C-element, but that it has not yet completed this development. The fact that the conditional interpretation (roughly, ‘in case’) has developed early, in both asyndetic and syndetic protases, but that the use in asyndetic protases becomes more frequent in PDG, is an argument in favor of such an ongoing reanalysis.

Third, a potential counterargument is the fact that a complementizer sollte would take a bare infinitive as its complement, and that there are no comparable C-elements in German: There are either complementizers (like dass ‘that’, wenn ‘if’, falls ‘in case’, …) taking a finite verb, or complementizers taking a zu ‘to’-infinitive (like um ‘in order to’ or anstatt ‘instead of’). Given the semantic, and partially distributional, similarity to falls (Reis & Wöllstein 2010), however, it is not expected that a complementizer-sollte would compete with non-finite complementizers. Rather, one may expect the lexical verb eventually to become finite. A transitional period with a bare infinitive would not be so unusual. There are relevant parallels in other languages where formerly finite auxiliaries undergo grammaticalization. Estonian for instance expresses negation by an invariate negative auxiliary that has historically lost all its finiteness features (unlike other Uralic languages, such as Finnish or North Sámi, where the negative auxiliary still inflects for person and number), followed by the infinitive of the main verb. That is, the loss of finiteness features on the grammaticalising auxiliary has not (yet) led to the expression of person and number on the lexical verb in Estonian. Under the cartographic approach, finiteness is represented high, in the lowest head of the C-domain (Rizzi 1997). This is above the domain of mood and modality. It is therefore theoretically not impossible for a head being reanalysed from Mood_irrealis to a head in the C-domain to continue to express finiteness information. Eventually, an emerging complementizer sollte would cease to carry the finiteness features of the clause. These would then be realised on the verb that was originally selected by the modal.

Interestingly, it is not hard to find typos or performance errors that could be interpreted as sollte having been reanalysed as a (conditional) complementizer, as there is a second finite verb, like wird in (25).20

Such apparent performance errors are a further indication that a reanalysis of sollte as a conditional complementizer is already underway in German.