系动词Be(am, is, are)的用法
基本形式：am、are、is
1、am
第一人称I+am (注意：“I”无论何时都要大写，不管位于句首还是句中).
例：I am a beautiful girl. I am twenty.
I am 还可缩写成I'm。 如：I'm David. 我是大卫。
但是am与not不能缩写。
2、are
第二人称you+are; 一人称复数we+are; 三人称复数they+are 其他复数名词+are。
(注意：第二人称单复数同形，都为you)
例：You are my good friend. We are in the same class.
They are on the road. The books are on the desk.
are与主语还可缩写。如：
We are= We're， They are =They're, You are = You're。而are与 not可缩写成aren't。
3、is
第三人称he(男) +is ; she(女)+is; it(不是人)+is;其他单数名词+is。
例：HeShe is a good student. It is a white cat. A dog is on that street.
is也可与主语缩写,如:
He is = He's, My mother is = My mother's等。但是This is不可缩写。而is与not可缩写成 isn't。
如:This isn't a book. 这不是一本书。