English modality refers to modal verbs, which are in the category of auxiliary verbs. The ten modals differ from the other auxiliaries because modals do not have tense. To illustrate this, "be" and "do" and "have" are auxiliaries and have tenses, thus they are also used as main verbs [to be, infinitive: be, been, being; to do, infinitive: do, did, does, doing; to have, infinitive: have, had, has, having].
Auxiliary modal verbs, or simply modals, have two unique functions. One is that they form conditionals. The other is that they express the mood of a verb, thus they express such moods as necessity (e.g., can, will), ability (e.g., can, may), possibility (e.g., may, could), probability (e.g., might, ought), certainty and uncertainty (e.g., will, shall; will not, shall not).
The ten modals are: can, could, may, might, must, ought, shall, should, will, would. Some sources may also include one or all of these: need to, used to, have to. These are not true modals but have taken on some modal characteristics through colloquial usage.
- Modals are invariable, meaning they take no conjugation.
- Modals must be used with a main verb: in the preceding sentence, "must" is the modal and "be" is the main verb.
- Modals must always be used with the infinitive base form of the verb: e.g., to run, base: run.
- Modals in statements follow the Subject Verb Object/ Compliment (SVO/C) sentence order: You (S) must (modal) go (V) home (O).
- Modals in questions follow the inverted question sentence order with a split verb MSVO/C: Must (modal) you (S) go (V) home (O)?
- Modals may, can, and will have past forms: might, could, would.
- Modals used with a present participle speak of possibility or probability in the future: She might be willing to be there.
- Modals used with a past participle speak of things that did not happen in a past time: You should have warned me that he was there.
- Modals suit mixing all time periods: e.g., I may think (present) she will wish (future) he might have done so (past) sooner than he did (further past).
- Modals take negative contractions, e.g., can + not = can't.
- Modals express advice, expectation, authority, caution, permission, requests, politeness, options, hypothetical, obligation, habitual situations, preferences, etc.
- Modals are preceded by wh-words in wh-questions: e.g., Where (wh-word) will (modal) Sally (S) go (V)?
- Modals express conditionals with a conditional If clause and a resulting then clause: If you had won, then you would not have met them.