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 this, not just that you are not required to do this. 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 this; You needn't do this. The above negative forms are not usually used in the sense of a factual conclusion; 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 of must in tag 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 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 above) is sometimes given the eye dialect spelling oughtta. 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 grammatically 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,