Irrealis and aspect

The aspectual parameters perfective/imperfective and actual/previous—described above for present and past realis—also apply for irrealis status, marked by a modal. For example (illustrating with verb live):

The exceptions appear to be that modal can and semi-modal is able to are not used with imperfective or with previous aspect. That is, one would not say, for example, *can be doing, or *is able to have done; could is used instead—could be doing, could have done.

However, past tense does not apply to irrealis when expressed by a modal (save for the scheduled activity modal, be to), except in the context of back-shifting.

Modals and semi-modals differ in that a modal cannot follow another verb, whereas a semi-modal may occur anywhere within a sequence of verbs. A further difference is that semi-modals behave like regular verbs in showing present and past forms; thus, alongside has to live, has to have lived, etc., we get had to live, had to have lived, etc. Notwithstanding these grammatical differences, modals and semi-modals are linked to the same set of modal categories, within irrealis status.