am is 的过去式为was are的过去式为were.
当主语是第一人称单数或第三人称单数时，谓语动词用was
主语是第二人称或其他人称复数时，谓语动词用were。
肯定句：主语+was (were) +表语+其他
I was late yesterday. (昨天我迟到了。)
否定句：主语 + wasn't／ weren't + 表语 + 其他
并且was， were与not可以缩写成wasn't， weren't。
I was not (＝wasn't) here yesterday． 昨天我不在这儿。
My parents were not (＝weren't) at home last Sunday．上周日我父母不在家
一般疑问句：Was(Were) + 主语 +表语 + 其他
把was， were提到句首，句末用问号即可变为一般疑问句
肯定回答用“Yes， 主语+was／were．”
否定回答用“No，主语+wasn't／weren't．
Were you ill yesterday? (你昨天病了吗)
肯定回 Yes， I was. (是的，我病了。)
否定句： No， I wasn't. (不，我没病。)
