be动词用法：
当做主动词时，“be”在性质上属于接系动词(The Linking Verb), 后面要有名词、形容词、地方副词或短语作补足语(The Complement)。例如：
The man is a science teacher.
这个男子是一位科学教师。
Mary's new dresses are colourful.
玛丽的新衣服色彩鲜艳。
I have been there before.
我以前去过那里。
My mother is in the kitchen now.
我妈妈现在在厨房里。
这四个都是陈述句，可以变成疑问句，方法简单，把主语和“be”或助动词对换位置即可：
Is the man a science teacher?
Are Mary's new dresses colourful?
Have I been there before?
Is mother in the kitchen now?
当“be”要在祈使句中出现时，陈述句可借助助动词“do”，否定句必须在前加上助动词“don't”，如：
Don't be silly!
Do be obedient!
Be careful!
Don't be a fool!
“Be”有两种缩写法，如下：
He's not...../He isn't....
You're not...../You aren't...
但“am + not”的缩写法只有一个：
I'm not.