There are only twelve modal auxiliary verbs, but they are used with very great frequency and in a wide range of meanings. They express concepts or attitudes relating to recommendation, obligation, necessity and prohibition; permission and refusal; possibility, expectation, probability and certainty; promise and intention; ability and willingness. The set of twelve verbs consists of four paired forms - can, could; may, might; shall, should; will, would; and four single forms - must, ought, need, dare. There are no other forms, and all modals are therefore, to varying degrees, 'defective' verbs. The two verbs need and dare present special problems: dare can follow the grammatical patterns of either modal auxiliaries or lexical, 'regular' verbs, while need contrasts grammatically with the regular verb to need. The principal distinctive formal features of modal verbs are , explicitly: a. negative sentences are formed by adding not after the modal verb; b. interrogative sentences are formed by inverting the subject and the modal verb; c. there is no -s ending in the third person singular in the present tense, as there is with regular verbs; d. the modal verbs are followed by the infinitive of a verb without to (except in the case of ought). All these features contribute to the distinction between the two grammatical patterns of dare, and between the modal verb need and the regular verb to need: Regular Modal He dares to go. ______ needs He doesn't dare to go. He dare not go. need need Does he dare to go- Dare he go? need Need Note that the modal verb need, and dare in its modal pattern, are used only in negative and interrogative sentences. The modal verbs have no infinitive forms, and no participle form. Hence the need on occasion for a number of more or less synonymous expressions having a fuller range of terms - be able to, or have to, for example. It would not be possible to use can or must in the following: - I'd like to be able to speak English fluently. (infinitive) - No one has been able to solve the problem. (present perfect) - I'm having to read this very carefully. (present continuous) The modal verbs are also limited in their range of time reference. When used with the 'present' infinitive of the main verb, they generally have a present or future time reference: can or could may or might He will or would help you e.g. immediately, later. shall or should must ought to. The use of the alternatives could, might, would, should, suggests a more tentative attitude on the part of the speaker. In requests, it represents what is commonly called the 'polite' form: 'Would (or Could) you help me?' This particular use of could, might, would, should, is consistent with their appearance in either of two forms of conditional sentences: could might I think he would help you if you tell him your problem. should Ability or potential: can, could, be able to Can is used to indicate (1) the possession of ability in general, or (2) being in a
Graver, B.D. 1996. Advanced English Practice. With Key. London: Oxford University Press.