could

The modal verbs of English are a small class of 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, must, shall, should, will and would. Certain other verbs are sometimes, but not always, 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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