what as subject of a clause. When what is the subject of a clause, it may take a singular or plural verb, depending on the sense. What is singular when taken as the equivalent of that which or the thing which: I see what seems to be a dead tree. It is plural when taken as the equivalent of those which or the things which: He sometimes makes what seem to be thoughtless mistakes.
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what in a clause that is the subject. When a clause that has what as its subject is itself the subject of a sentence, it may take a singular or plural verb, but how you decide this is more complicated. Most of these what clauses are singular: What they always wanted was a home of their own. In fact, what clauses are usually singular even when the verb is a linking verb, such as be or seem, followed by a plural noun or a series of nouns: What she kept in her drawer was ten silver dollars. What truly commands respect is a large air force and a resolute foreign policy.
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In some cases, you can treat a clause with what as the subject as singular or plural, depending on the emphasis you want to convey. In What excite him most are money and power, the implication is that money and power are distinct elements; in What excites him most is money and power, the implication is that money and power are a single entity.
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In other cases, what clauses are plural. The what clause as a whole is plural if it has a plural verb: What seem to be two dead trees are blocking the road. The what clause is usually plural if the verb in the main clause is a linking verb followed by a plural noun or noun phrase: What most surprise me are the inflammatory remarks at the end of his article.
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There are also certain sentences that have a main verb followed by a plural noun or noun phrase whose sense requires that the what clause be plural, as in What traditional grammarians called “predicates” are called “verb phrases” by modern linguists and What the Romans established as military outposts were later to become important trading centers. In these sentences, the plural nouns predicates and outposts give the what clauses their plural meaning.
