I found a sentence on an English grammar book. I am confused why an article is used before a plural.
Each of the following sentences is preceded by a bare infinitive.
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It is only the indefinite article a/an which cannot be used with plurals. That word is a worn-down form of one and is used only with singular nouns. The equivalent determiner with plurals is the "empty" article (designated in linguistic texts with '∅')—that is, no article at all.
But it is perfectly OK to use the definite article the with plural nouns. The has no 'number': it is both singular and plural. It is used (in very general terms) to 'mark' an entity or collection of entities as definite: known, familiar, or in some way distinguished from the totality of similar entitities.