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1. It is nice to meet you.

2. It was nice to meet you.

3. It has been nice to meet you.

4. It is nice meeting you.

2. It was nice meeting you.

3. It has been nice meeting you.

Which of the above sentences is not acceptable?
Comments  
All are correct and common.
Nice to meet you” normally stands for “It IS nice to meet you,” and is therefore said at the time of meeting (typically while shaking hands, etc.), while "nice meeting you" is most commonly used to mean “It HAS BEEN/WAS nice meeting you,” i.e. just before parting, to repeat/ reinforce the sentiment earlier expressed by “nice to meet you.”

Nice to meet you stands for the present tense.

Nice meeting you stands for the present perfect or past tense.

Why?
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Hello Teo

I think your own post answers the question: by ellipsis.

(Or have I misunderstood?)

MrP
MrPedanticI think your own post answers the question: by ellipsis.
1. It is nice to meet you.

2. It was nice to meet you.

3. It has been nice to meet you.

4. It is nice meeting you.

5. It was nice meeting you.

6. It has been nice meeting you.

If #2 and #3 are correct, why can't "nice to meet you" stand for "it was/has been nice to meet you"?

If #4 is correct, why can't "nice meeting you" stand for "it is nice meeting you"?
Hello Teo

I'm not sure I would find it odd if someone said "Nice meeting you!" instead of "Nice to meet you!", or vice versa.

(Unfortunately British people are more likely to say "Hello", make a little mouth-and-eyebrows movement, and shuffle about awkwardly, when introduced to each other. But I'll listen out for it and let you know.)

MrP
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Why do you continue to ask the same question repeatedly, Teo? Well, I've had about enough of all this. Nice having met you all.

MM