Be动词有：be； is, am,are, (一般现在时，时态的be动词)

was, were, （过去时态的be动词）

being,（现在分词）

been; （过去分词）

1. The man is back.
2. They are back.
3. He was back.
4. They were back.
5. They have been back.
6. be动词的用法
7. 后面接名词，形容词，地点副词或短语作补足语。
8. 1.The man is a teather.(名词补足语，补充说明)
9. 2.Mary\'s new dresses are colorful.(形容词补足语，补充说明)
10. 3.My mother was in the kitchen.（地点副词补足语）
11. Be动词的练习
12. 1.他们是老师（they,teachers）(现在时)
13. 2.他以前是一名老师（he,a teacher，before）（过去时）
14. 3.他已经当了3年的老师。（for 3years）（完成时）

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答案：1.They are teachers.

2.He was before a teacher.

3.He has been a teacher for 3years.