Contents introduction chapter I grammatical meaning, grammatical form

Must and had to. The modal must expresses obligation or necessity: You must use this form; We must try to escape. It can also express a confident assumption (the epistemic rather than deontic use), such as in It must be here somewhere.
An alternative to must is the expression had to (in the present tense sometimes have got to), which is often more idiomatic in informal English when referring to obligation. This also provides other forms in which must is defective and enables simple negation When used with the perfect infinitive (i.e. with have and the past participle), must expresses only assumption: Sue must have left means that the speaker confidently assumes that Sue has left. To express obligation or necessity in the past, had to or some other synonym must be used.
The formal negation ofmustismust not (contracted to mustn't). However the negation effectively applies to the main verb, not the modality: You must not do this means that you are required not to do it, not just that you are not required doing it. To express the lack of requirement or obligation, the negative of have to or need (see below) can be used: You don't have to do it; You needn't do it.
The above negative forms are not usually used in the sense of confident assumption; here it is common to use can't to express confidence that something is not the case (as in It can't be here or, with the perfect, Sue can't have left).
Mustn't can nonetheless be used as a simple negative ofmustintag questions and other questions expressing doubt: We must do it, mustn't we? Mustn't he be in the operating room by this stage?
Ought to and had better. Ought to is used with meanings similar to those of should expressing expectation or requirement. The principal grammatical difference is that ought is used with the to-infinitive rather than the bare infinitive, hence we should go is equivalent to we ought to go. Because of this difference of syntax, ought is sometimes excluded from the class of modal verbs, or is classed as a semi modal. The reduced pronunciation of ought to (see Contractions and reduced pronunciation above) is sometimes given the eye dialect spelling ought to¹⁶.
Ought can be used with perfect infinitives in the same way as should (but again with the insertion of to): you ought to have done that earlier.
The negated form is ought not or oughtn't, equivalent in meaning to shouldn't (but again used with to). The expression had better has similar meaning to should and ought when expressing recommended or expedient behavior: I had better get down to work (it can also be used to give instructions with the implication of a threat: you had better give me the money or else).
The had of this expression is similar to a modal: it governs the bare infinitive, it is defective in that it is not replaceable by any other form of the verb have, and it behaves syntactically as an auxiliary verb. For this reason the expression had better, considered as a kind of compound verb, is sometimes classed along with the modals or as a semi modal.
The had of had better can be contracted to'd, or in some informal usage (especially American) can be omitted. The expression can be used with a perfect infinitive: you'd better have finished that report by tomorrow. There is a negative form hadn't better, used mainly in questions:¹⁷ Hadn't we better start now? It is
¹⁶ David Rubin, "might could (double modal)", The Mavens' Word of the Day, Random House, November 20, 2000,
p. 301.
¹⁷Warner, Anthony R., English Auxiliaries, Cambridge Univ. Press, 1993, p. 144.
more common for the infinitive to be negated by means of not after better: You'd better not do that (meaning that you are strongly advised not to do that).
Used to. The verbal expression used to expresses past states or past habitual actions, usually with the implication that they are no longer so. It is followed by the infinitive (that is, the full expression consists of the verb used plus the to-infinitive). Thus the statement I used to go to college means that the speaker formerly habitually went to college, and normally implies that this is no longer the case.
While used to does not express modality, it has some similarities with modal auxiliaries in that it is invariant and defective in form and can follow auxiliary-verb syntax: it is possible to form questions like Used he to come here? and negatives like He used not (rarely usedn't) to come here. More common, however, (though not the most formal style) is the syntax that treats used as a past tense of an ordinary verb, and forms questions and negatives using did: Did he use(d) to come here? He didn't use (d) to come here.
The verbal use of used to should not be confused with the adjectival use of the same expression, meaning "familiar with", as in I am used to this, we must get used to the cold. When the adjectival form is followed by a verb, the gerund is used: I am used to going to college in the mornings.