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