What is the abbreviation for have to? 

have to

The English modal verbs are a subset of the English auxiliary verbs used mostly to express modality (properties such as possibility, obligation, etc.). They can be distinguished from other verbs by their defectiveness (they do not have participle or infinitive forms) and by their neutralization (that they do not take the ending -(e)s in the third-person singular). The principal English modal verbs are can, could, may, might, shall, should, will, would, and must. Certain other verbs are sometimes classed as modals; these include ought, had better, and (in certain uses) dare and need. Verbs which share only some of the characteristics of the principal modals are sometimes called "quasi-modals", "semi-modals", or "pseudo-modals".

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