Be动词的否定
在 am ， is ， are ， was ， were 后面加not.
缩写式：am not ， isn’t ， aren’t ， wasn’t ， weren’t
1、 The man isn’t back. 主系表结构
2、 I am not back.
be动词根据主语决定时态，单复数
3、 They aren’t back.
4、 He wasn’t back.
5、 They weren’t back
Be动词提问和回答
Is he a teacher ?
Yes ， he is. / No ， he isn’t.
Are you a teacher?
Yes ， I am / No ， I am not.
Were they teachers?
Yes ， they were . / No ， they weren’t.
