[a] They can most easily be distinguished from other verbs by their defectiveness (they do not have participles or plain forms[b]) and by their lack of the ending ‑(e)s for the third-person singular.
A list of what tend to be regarded as modal auxiliary verbs in Modern English, along with their inflected forms, is shown in the following table.
Descriptive grammars of English differ slightly on the criteria they set for modal auxiliary verbs.
Other than in the present tense, even lexical verbs lack subject agreement and so this test is inapplicable to either had better or used.
[21] Among these five verbs, The Cambridge Grammar selects the pair can and will (with could and would) as "the most straightforward of the modal auxiliaries".
The aforementioned Old English verbs cunnan, magan, sculan, and willan followed the preterite-present paradigm (or, in the case of willan, a similar but irregular paradigm), which explains the absence of the ending -s in the third-person present forms can, may, shall, and will.
This was another preterite-present verb, of which moste was in fact the preterite (the present form mot gave rise to mote, which was used as a modal verb in Early Modern English, but must has now lost its past connotations and has replaced mote).
Similarly, ought was originally a past form—it derives from ahte, preterite of agan ("own"), another Old English preterite-present verb whose present tense form, ah, has also given the modern (regular) verb owe, and ought was formerly used as a preterite form of owe.
[40] The preterite forms given above (could, might, should, and would, corresponding to can, may, shall, and will, respectively) do not always simply modify the meaning of the modal to give it past reference.
All the preterites are used as past equivalents for the corresponding present modals in indirect speech and similar clauses requiring the rules of sequence of tenses to be applied.
This "future-in-the-past" (also known as the past prospective) use of would can also occur in a main clause: I moved to Green Gables in 1930; I would live there for the next ten years.
The modal would (or should as a first-person alternative) is used to produce the conditional construction typically used in clauses of this type: If you loved me, you would support me.
"[46] The modal verb can expresses possibility in a dynamic, deontic, or epistemic sense, that is, in terms of innate ability, permissibility, or probability.
It is preferable to use could, may or might rather than can when expressing likelihood in a particular situation[citation needed] (as opposed to the general case, as in the "rivalry" example above, where can or may is used).
[55] The verb may expresses possibility in either an epistemic or deontic sense, that is, in terms of probability or permissibility.
For uses of might in conditional sentences, and as a past equivalent to may in such contexts as indirect speech, see § Preterite forms above.
A less common use of may is optative (to express a wish), as in May you live long and happy[56] (see also English subjunctive).
May have indicates uncertainty about a past circumstance, whereas might have can either have that meaning or refer to possibilities that did not occur but could have (see also conditional sentences above).
As for any modal auxiliary, a negative interrogative (Shouldn't you check your credit card statement?)
Hendrik Poutsma writes that "The force of must, notably that of representing the subject under pressure of an overmastering desire [. . .
"[68] Examples of the pair are: Ought differs from the central modal auxiliary verbs both in taking as its complement a to-infinitival rather than a bare infinitival clause (compare He should go with He ought to go) and in lacking a preterite.
[73] The use of ought as a lexical verb as in They didn't ought to go is generally thought of as restricted to nonstandard dialects[74] but has been described as also sometimes found in informal standard usage.
What this censure suggests is that lexical ought with periphrastic do is a well-established usage in colloquial [British English].
It is most commonly used here in the negative, meaning that an action was (from the present perspective) not in fact necessary: You needn't have left that tip.
Hendrik Poutsma adds: I had as lief (or lieve), although now antiquated and mostly replaced by I had as soon, has never fallen completely into disuse. . . .
[102]During the second half of the 20th century, the frequencies of use of both the modal auxiliary verbs and of alternatives to them showed considerable change.
A comparison[103][104] of the frequencies in the British corpora LOB and FLOB (with material from 1961 and 1991 respectively), and of those in the American corpora Brown and Frown (1961 and 1992 material respectively) shows: (Percentage changes shown in parentheses come with χ2 values of greater than 0.05; they are of less statistical significance.)
A study of modal auxiliary verbs and quasi-modals in American, British and Australian examples (given equal weight) of a variety of genres of written and spoken English in the 1990s found[105] that the totals were: Commenting on a different but similar set of figures, Longman Grammar of Spoken and Written English observes of ought, need, dare, and use /jus/: In view of the considerable attention given to these marginal auxiliaries in grammatical descriptions of English and English language teaching materials, it is worth noting how rare they are, particularly in negative and interrogative auxiliary constructions.
Hence, a modal may introduce a chain of verb forms in which the other auxiliaries express properties such as aspect and voice, as in He must have been given a new job.
If infinitival to is regarded as an auxiliary verb, then longer chains are possible, as in He must have been encouraged to try to serve tea.