不定冠词a (an)与数词one 同源，是"一个"的意思。a用于辅音音素前，一般读作[e]，而an则用于元音音素前，一般读做[en]。

1) 表示"一个"，意为one；指某人或某物，意为a certain。

A Mr. Ling is waiting for you.

2) 代表一类人或物。

A knife is a tool for cutting with.

Mr. Smith is an engineer　　3) 词组或成语。

a little / a few / a lot / a type of / a pile / a great many / many a / as a rule / in a hurry / in a minute / in a word / in a short while / after a while / have a cold / have a try / keep an eye on / all of a sudden