Have you met before?
你们以前见过面吗?

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如果是三个人在社交的场合, 有时候你同时认识其它两人, 但是还不确定他们两个人彼此之间认不认识, 这时候你就应该先问问, "Have you met before?" 要是他们彼此没见过, 你就要负责介绍他们认识. 如果是只有两个人的情况, 而你不确定你跟对方之前有没有见过面, 这时最好主动先说, "Have we met before?" (我们以前见过面吗?)

如果两人还不认识, 就说Nice to meet you. 很高兴认识你.

如果发现两个人原来早就认识, 请用 I didn't recognize you!.解释如下：

I didn't recognize you! 我都不认出你了!

如果两人讲讲话突然发现对方是自己失散多年的好友的话, 你就可以很惊讶地说, "I didn't know it was you!" (我不知道原来就是你!) 不然就是 "I didn't recognize you!" (我都认不出你了!) 要是你认得某人, 但他一副不认识你的样子, 这时候你则可以说, "Hey! Don't you recognize me?" (喂! 你不认得我了吗?)
这个 recognize 在这里是当"认出来"的意思, 跟 know "知道"是不一样的意思. 例如有人化装化很浓, 你都认不出她了, 你就可以说, "I don't recognize you!" 但你不能说, "I don't know you." 这两者是不一样的.

Have you met before?
你们以前见过面吗?
I didn't recognize you!
我都不认出你了!
I didn't know it was you!
我不知道原来就是你!

Have you met before?
你们以前见过面吗？
Have we met before?
我们以前见过面吗？
I didn't recognize you!
我都认不出你了！
Hey! Don't you recognize me?
嘿！你不认得我了吗？

Have you met before?

Have we met before?
I didn't know it was you!
I didn't recognize you!

Have you met before?
你们以前见过面吗？
Have me met before?
我们以前见过面吗？
I didn't recognize you!
我都认不出你了！
Hey! Don't you recognize me ?
喂，你不认识我了吗？

Have you met before?
你们以前见过面吗?

Have you met before?
你们以前见过面吗?
I didn't recognize you
我都认不出你了

have you meet before?

Have you meet before?
I didn't recognize you.
I didn't know it was you.