I am back也是系表结构.back是副词
可以作表语的词：
1、名词：
I'm a student.
She is a doctor.
2、代词、数词：
I think the thing is this.
How much I want is three.(我想要的数量是3）
3、形容词（最常见）：
People are equal.
He is happy.
Everything is still(静止的）.
You are beautiful.
4、副词（比较特殊,仅限于back,in, on, off, out, away, behind, up, down,around, round, below, inside, outside,here, there, upstairs, downstairs一些表方位、方向的词）
I must be off now.（我现在必须走了）
-Where is he? -He is upstairs.
The boy is there.
5、介词短语：
I am at home now.
The students are in the classroom.
6、现在分词、过去分词:
The book is interesting.
My mother is not excited at the news.
I'm very tired.
7、动词不定式to do:
My job is to teach.
My dream is to become a doctor.
8、动名词：
Seeing is believing.(眼见为实）
His wish is buying his mother a car.
9、从句：
The problem is how we can solve the problem.
The fact is that he hit me.(事实是他打了我）
