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The word “could” is simply a verbal or verb-form also known as a modal auxiliary. Just like the auxiliary verbs “have “( to have) and “be” (to be: the copula), the modal auxiliary is used as part of a verbal phrase.
Thereof, what type of verb is could?
Could is used as both an auxiliary verb and a modal verb. Could is almost always used with a main verb.
Hereof, is Could an adverb?
In other words, an adverb describes, modifies or provides more information about a verb in a sentence. So, if you said "I am going to quickly run to the store," the adverb in that sentence (quickly) would be modifying the verb "run." In addition to verbs, adverbs also modify adjectives, other adverbs and word groups.
In English, the modal verbs commonly used are can, could, may, might, must, will, would, shall, should, ought to, had better, "have to" and sometimes need or dare.