Ana Werkmann Horvat - Layers of Modality: On Double Modal Constructions by the Example of Croatian

The judgement that John is studying and John might be studying have different meanings is quite uncontroversial. Even to a nave speaker, it is apparent that the former talks about a real event, something happening here and now (In other words, they can express either necessity or possibility, and a wide variety of modal flavours, such as epistemic (based on evidence), deontic (based on knowledge) or circumstantial (based on circumstances).

These expressions can be stacked on top of each other in sentences such as:

In examples (1) and (2) a modal verb (1) and (2) show how an adverbial modal can be combined with a verbal modal, examples (3) and (4) show how one type of verbal modality can embed another type of verbal modality. However, at least at first glance, the verbal-verbal combination seems to be possible only with a modal verb embedding a semi-modal. If we were to replace the semi-modals have to and be able to with their modal verb equivalents must and can, respectively, the structures no longer seem to be grammatical in standard English:

These two data points are certainly not new in the literature. While it is generally claimed that two modal verbs cannot combine together in standard English and that a modal verb can only embed a non-modal verb, it has been noted in the literature that double modal constructions (DMCs) are, in fact, possible in some non-standard dialects of Southern American English () as in (7):

They also appear in some non-standard Scottish English dialects (, 2015) in the form of DMCs that are similar to the structure in (7), and even triple modal constructions (TMCs) as in (8):

These structures are still highly stigmatised in English since they are considered to be a feature of certain non-standard dialects. For this reason, it is often hard to get grammaticality judgments on them from native speakers. Nevertheless, they have been a matter of interest for a wide variety of studies which have mainly studied this phenomenon from a sociolinguistic or syntactic perspective.

Cross-linguistically, however, English is an outlier, and yet the main body of research remains on DMCs and TMCs in English. Oddly, other languages seem to be fairly under-researched, even though DMCs are not rare, nor stigmatised, cross-linguistically, but rather quite standard. For instance, DMCs appear in Germanic languages such as Swedish, Icelandic, Danish, Dutch and German. Thrinsson & Vikner (1995) report the possibility of stacking modals in some Scandinavian languages, specifically Danish (example (9)) and Icelandic (example (10)):