I'd like to know what"you've seen yesterday" means in this context:
If you can talk with your friend who you haven't seen for a long time as if you've seen her/him yesterday, you're best friends.
I thought we can't use "have seen" with "yesterday" because this "have" means experience.
Short answer: Native usage on this point (at least in the US) is quite a bit looser than what prescriptive grammar books teach you.
In this particular case, the emphasis is on the fact that (hypothetically) you've seen your friend, that such an action has occurred. But, because the proximity is important, the completed task is not "seeing your friend" but "seeing your friend yesterday."
The specific point in time is secondary; we don't really care that it was at 9:00 or 17:00 or whenever yesterday. And, if you can think about it figuratively, it doesn't matter that it was specifically yesterday versus last Friday or a week ago or whenever. Yesterday here isn't to designate a specific time, just to imply proximity, almost like a synonym for recently.
This usage largely results from the fact that the example is hypothetical--it's the hypothetical nature that leads the speaker to not think of yesterday as a specific point in time. The indicative version, You've seen your friend yesterday sounds fairly unnatural, albeit not totally ungrammatical. It places the emphasis on the accomplishment of having seen your friend, and again, mentions yesterday almost as an afterthought.
That's the best explanation I can give as a native speaker--the given example doesn't sound odd to me at all. I know blaming prescriptive vs. descriptive grammar is a not very satisfying answer, but it's my best attempt to explain the psychology going on in this choice of tense.
This explanation is from the link attached below..
For example, saying “I have seen the movie yesterday” would be incorrect. The verb form is being used correctly (“have seen”), but the word “yesterday” refers to a specific moment in the past–it does not connect the past to the present..
So from above,
I think you should be using had instead of have or replace the word yesterday with before.
You have seen them before or You had seen them yesterday, both would be correct.
Read more here.
Present Perfect is a present tense - it describes the present state of something, as being connected to something completed (perfect).
Think about "your friend who(m) you haven't seen for a long time" as a description of present state of things.
In this sentence, you could think of "you've seen her/him yesterday" as an alternative state of things. "As if" connects these two alternatives.
This sentence can also be regarded as an idiom that would be weakened if the second part used past simple instead.
"You've seen her/him yesterday", by itself, does not clearly describe a present state, since it mainly indicates the time of the past "seeing". If that time is the focus of the sentence, then past simple should be used.
The difference between I saw (Past Simple) and I have seen (Present perfect simple) is that for present perfect simple there has to be some lasting effect.
I saw James in the park - no lasting effect
Have you seen John? - lasting effect if true... after seeing him, you would know that he is around
I have seen a doctor, and he told me... - lasting effect- the doctor told you something.
In your sentence, the point you are making is that talking to your best friend is like talking to somebody else (not your best friend) that you had seen yesterday. Seeing the not-best-friend yesterday would make it easier to talk to then today- it would have had some lasting effect, so it's appropriate to use Present Perfect Simple- "as if I had seen him yesterday".
Note that as if is a hypothetical situation, so you should really say "you had seen". See here for a discussion of this point.
-Have you been to the market yet? -Yes, I have. – When did you go there? – Yesterday.
When recent news is introduced we use present perfect. For details we use past simple. They say it's unusual to have present perfect with expressions of finished time.It is unusual but not impossible. They often occur in news items (announcing the news and giving details in th same clause ).
A 24-year-old soldier has been killed in a road accident last night.
The second version is:
"In spoken and written journalistic styles, the present perfect is sometimes used to stress the current relevance of events, even though definite past time adjuncts may be present."(Cambridge Grammar of English).
