Central and marginal modalsThe central modal verbs are can, could, may, might, must, shall, should, will, would. The marginal modal verbs, sometimes called semi-modal verbs, are dare, need, ought to, used to. All share the following characteristics: (1) They are auxiliary verbs. (2) They have no third-person -s form: She may go, They may go (contrast She goes, They go). (3) They have no non-finite forms (no infinitive, -ing participle, or -ed participle), and therefore in standard English can appear only in initial position in the verb phrase, and cannot occur with each other (although ‘double modal’ forms such as might could go occur in some non-standard varieties, such as Southern US English). (4) All except ought and used are followed by the bare infinitive without to. (5) They have idiosyncratic semantic and formal features, affecting particularly their use in the past tense and in negation.
Kinds of modals(1) Epistemic modals: may (He may be at home), might (It might get too hot), must (It must be your sister on the phone), ought to (They ought to have heard by now), should (The show should be over soon), will (That will be the doctor), would (Who would have guessed he was so young?). (2) Deontic modals: can (You can leave now), could (Could I go now please?), may (You may smoke), might (Might we have another one?), must (You must be patient), need (You needn't say anything), ought to (I ought to write more often), shall (You shall have my resignation letter tomorrow: a promise or a threat), should (You should write more legibly). (3) Dynamic modals: can (Neil can drive a car), could (He couldn't drive at that time), dare (I daren't tell/don't dare tell my parents), shall (We shall allow no obstacle to impede our programme), will (I will stay as long as I wish).
Future expressionsShall (with first-person subjects only, particularly in Southern England) and will (often contracted to 'll) express future time and are often said to comprise the future tense: I shall be back next week; He will be here soon. Other ways of expressing the future include the semi-auxiliaries (see below) be going to (It's going to rain) and be to (She is to be married tomorrow), the present continuous (I'm leaving for New York next week), and the simple present (The plane leaves at noon).
Marginal modals and semi-auxiliariesDare, need, ought to, and used to share most of the characteristics of modal verbs but are marginal for various reasons. Unlike the central modals, ought and used are followed by to and despite prescriptive objections often combine with do in negative and interrogative constructions, like a full verb: especially in England, They didn't ought to say that alongside the more traditionally acceptable They oughtn't to say that; Did he used to play the violin?, alongside the rare Used he to play the violin? Used to also differs semantically from central modals, since it conveys aspect (habitual situation) and not modality. In negative and interrogative contexts, dare and need may be either modals (I daren't object; Need I say more?) or full verbs with preceding do and following to-infinitive (I don't dare to object or the blend without the to, I don't dare objects; Do I need to say more?). Elsewhere, they are full verbs: I dare/dared to object; I need/needed to say more. There are a number of semi-auxiliaries that express modal or aspectual meanings, such as be able to, be about to, be bound to, be going to, have to, have got to. They can be used as non-finite forms and are therefore convenient substitutes for modals in non-finite positions: for example, the use of You may be able to see me tomorrow instead of the impossible *You may can see me tomorrow.
NegationWhen a verb phrase containing a modal is negated, the negation applies in some instances to the modal and in other instances to the proposition: for example, the modal is negated in You may not leave (You are not allowed to leave), whereas the proposition is negated in I may not be on time (It is possible that I won't be on time). The difference may affect the choice of the auxiliary: for example, epistemic must (It must be your sister on the phone) usually forms its negative equivalent through may not for negating the proposition (It may not be your sister on the phone: It is possible that it is not your sister on the phone) and can't for modal negation (It can't be your sister on the phone: It is not possible that it is your sister on the phone).
Past tenseCan, may, shall, will have the past-tense forms could, might, should, would. These forms are chiefly used to express tentativeness or conditionality rather than past time, so that there is no time difference between I may see you later and I might see you later, or between Can you pass the salt? and Could you pass the salt? The past forms, however, are used for past time in indirect speech (I may see you later is reported as She said that she might see me later). There are no past forms for must, dare, need, ought to. The epistemic modals indicate the past time of the proposition by using have: Andrew may/might have been in his office; You must have seen them; They will/would have landed by now. The deontic modals ought to have and should have express past obligation, usually with the implication that it was not fulfilled: You ought to have phoned (but you didn't); They should have come in. Dynamic could (was able to) and would (was willing to) are used for past time in negative contexts (He couldn't type; They wouldn't help us).
Would is commonly used in the main clause of a sentence expressing a hypothetical condition: If I were you, I would buy it; If you had seen them, you would have been shocked. Sometimes, in BrE, should is used with a first-person subject in place of would: If I had seen them, I should have reported it. Would and should appear in other hypothetical contexts: I was at the demonstration, but it would take too long to tell you what happened. Should may also appear after evaluative expressions (It's odd that he should say that; also It's odd that he says that) and expressions of necessity, intention, and the like (We insisted that he should stay). See DIALECT (UNITED STATES), SCOTS.
"MODAL VERB." Concise Oxford Companion to the English Language. . Encyclopedia.com. (August 5, 2019). https://www.encyclopedia.com/humanities/encyclopedias-almanacs-transcripts-and-maps/modal-verb
"MODAL VERB." Concise Oxford Companion to the English Language. . Retrieved August 05, 2019 from Encyclopedia.com: https://www.encyclopedia.com/humanities/encyclopedias-almanacs-transcripts-and-maps/modal-verb
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