A and an

Articles in the English language are the definite article the and the indefinite articles a and an. Use of the definite article implies that the speaker assumes the listener knows the identity of the noun's referent (because it is obvious, because it is common knowledge, or because it was mentioned in the same sentence or an earlier sentence). Use of an indefinite article implies that the speaker assumes the listener does not have to be told the identity of the referent. In some noun phrases, no article is used.

The rules of English grammar require that in most cases a noun, or more generally a noun phrase, must be "completed" with a determiner to clarify what the referent of the noun phrase is. The most common determiners are the articles the and a(n), which specify the presence or absence of definiteness of the noun. Other possible determiners include words like this, my, each and many – see English determiners. There are also cases where no determiner is required, as in the sentence John likes fast cars. Or the sentence Bob likes cool trains.

The definite article the is used when the referent of the noun phrase is assumed to be unique or known from the context. For example, in the sentence The boy with glasses was looking at the moon, it is assumed that in the context the reference can only be to one boy and one moon. However, the definite article is not used:

The indefinite article a (before a consonant sound) or an (before a vowel sound) is used only with singular, countable nouns. It indicates that the referent of the noun phrase is one unspecified member of a class. For example, the sentence An ugly man was smoking a pipe does not refer to any specifically known ugly man or pipe.

No article is used with plural or uncountable nouns when the referent is indefinite (just as in the generic definite case described above). However, in such situations, the determiner some is often added (or any in negative contexts and in many questions). For example:

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