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has是have的第三人称单数形式，比如he，she就要用has，而I，we，they要用have。针对had来说，have和has是现在时，had则是has和have的过去式。
have、had、has的用法和区别：
1、have、had、has区别在于不同时态和不同人称之间，三个单词其实表达为一个意思。
2、如果是一般现在时，第三人称单数的时候用has，其它（如第一人称和第二人称）用have。
3、如果是一般过去时，全部都用had。
4、has/have been是现在完成时，has用于第三人称单数，had been过去完成时，跟前者相比，比前者更为过去。
have
1、Alex has already gone
亚历克斯已经走了。
2、When I met her, she had just returned from a job interview
我遇见她时，她刚参加完一场求职面试回来。
3、You haven't sent her away, have you?
你还没有把她送走，是吗？
4、He arrived in San Francisco, having left New Jersey on January 19th
1月19日离开新泽西后，他到了旧金山。
5、I went out and had a walk around
我出去在周围散了一会儿步。
had
1、Had he succeeded, he would have acquired a monopoly
要是他当时成功了，他就会取得垄断地位。
2、If your customer thinks he's been had, you have to make him happy.
要是顾客感觉自己当了冤大头，你就得设法让他消气。
3、Unless she loses some weight, she's had it
除非她减点儿肥，否则她就没戏了。
4、I've had it. Let's call it a day
我受不了了。今天就到这儿吧。
5、Had I known what the problem was, we could have addressed it.
如果我当时知道问题出在哪里，我们就可以设法解决了。