You have been replaced by another person.
You were replaced by another person.
You are replaced by another person.
If something happens in recent past, which tense should I use?
What is the meaning of the above three sentences?
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All of your sentences are grammatically correct. Which you use depends on context: why you are saying this.
1.You are replaced by Bob.
2.You have been replaced by Bob.
Both of these are cast in the present tense (have, are) and address your current status. Either might be used to inform you, for the first time, of your new status, or if you already know that you are moving to a new position to inform you of your successor. They mean essentially the same thing. The version with the perfect construction (you have been replaced) would be the more natural in most circumstances; that with simple present (you are replaced) has a decidedly stiff and formal ring.
I regret to inform you that senior management have made their determination: you are replaced by Bob. Please clear your desk by noon and surrender your pass to Security.
However, the simple version is more natural if your informant is describing multiple changes:
Sally moves over to Sales, you replace her as section head, you are replaced by Bob, and we're authorizing a new hire to replace Bob. Congratulations!
The simple present may also be used with future or contingent rather than present reference:
Under the new scheme, you move up to section head; you are replaced by Bob.
The perfect construction cannot be used here.
3.You were replaced by Bob.
This version is cast in the past tense (were) and addresses a completed event. It would not ordinarily be used to inform you of something new; rather, it recalls the event in order to say something new about its context:
You were replaced by Bob because we were unhappy with your handling of the Adams account.
We needed to free you up for fulltime work on the Bergson account, so you were replaced by Bob on the Adams account.
The most common tense choices for talking about the recent past are present perfect and past simple. Your first two example sentences are in these tenses, while the third is simply wrong.
You have been replaced by another person
The tense here is present perfect, which is often used to talk about the very recent past. This is the most natural way to describe something that has just happened. This usage is also much more common in the UK than in the US.
You were replaced by another person
Here the tense is past simple. Past simple can be used to talk about the recent past, however the present perfect tense is often a better choice because the implicit timeline is more clear.
You are replaced by another person
This sentence is a mixed tense and would not be used by a native speaker. There are a number of ways to correct it, the simplest being to replace "are" with "were" to make the sentence past simple.