Most linguists agree that there are 10 core or central modals in English: can, could, may, might, must, ought, shall, should, will, and would. Other verbs--including need, had better and invariant be--may also function as modals (or semi-modals).
- Habitual Past
- Lexical Modality
- Marginal Modal
- NICE Properties
- Present-Day English (PDE)
- Primary Verbs
- Putative Should
- Sequence of Tenses (SOT)
- Shall and Will
- Verb Phrase
Etymology:From the Latin, "measure"
Examples and Observations:
- "When I was younger I could remember anything, whether it happened or not."
- "[G]overnment of the people, by the people, for the people, shall not perish from the earth."
(Abraham Lincoln, The Gettysburg Address, 1863)
- "There are many things that we would throw away if we were not afraid that others might pick them up."
- "A modal auxiliary has the following characteristics:
- Takes negation directly (can't, mustn't).
- Takes inversion without DO (can I? must I?).
- 'Code' (John can swim and so can Bill).
- Emphasis (Ann COULD solve the problem).
- No -s form for third-person singular (*cans, *musts).
- No non-finite forms (*to can, *musting)
- No co-occurrence (*may will)
(Jennifer Coates, The Semantics of the Modal Auxiliaries. Routledge, 1983)
- "As early as Old English, a group of verbs signaling modal characteristics of action share morphosyntactic and semantic features which later result in the formation of the category of modal auxiliaries. . . .
"The most important syntactic developments which distinguish [modals] from other verbs are the following: (1) they lost their non-finite forms and their ability to take non-verbal objects; (2) the preterite forms came to be used in the present, future or timeless contexts; (3) they did not develop the to- link with an infinitive (in the Southern standard); (4) they became more and more uncommon in contexts where they were not followed by an infinitive."
(Richard M. Hogg, et al., The Cambridge History of the English Language: 1476-1776. Cambridge Univ. Press, 1999)
- Modals and the Subjunctive
"Modals are also used where some languages would use the subjunctive mood. The Modern English subjunctive is very restricted and examples are given in (11a) and (12a). Alternatives using modals are provided in (11b) and (12b):
(11a) They insisted that he go. (subjunctive mood)Since subjunctives are not common in Modern English, I will not go into this more deeply."
(11b) They insisted that he should go.
(12a) I wish it were Friday. (subjunctive mood)
(12b) I wish it would be Friday.
(Elly Van Gelderen, An Introduction to the Grammar of English, rev. ed. John Benjamins, 2010)