1、不定冠词有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。

2、表示一个，意为one；指某人或某物，意为a certain。 A Mr. Ling is waiting for you.

3、代表一类人或物。 A knife is a tool for cutting with. Mr. Smith is an engineer.

4、词组或成语。 a little / a few / a lot of / 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