tenses

Dear saarthak299,

I'm happy to respond.

The simple past tense has the connotation that an action started and finished in the past.
1) I was a college student for four years.
That's a true statement; my college days started and ended a long time ago (before there was a think called the "internet"!)

The present perfect has the connotation that, even though the action started in the past, either the action itself still continues or some effect of the action is still present to us:
2) I have been a NY Mets fan for many years.
Another true statement: that sentence contains the true implication that I am still a NY Mets fan verily unto this day.

Both of those versions could be grammatically correct: they simply describe very different scenarios.
3) Not for nothing did he prosper as a business consultant since his arrival in American 20 years ago.
Grammatical & logically correct for a scenario in which the person's business career is over, perhaps because he retired or because he died. What's very clear from this version is that this work is in the past and no longer continues at the present time.
4) Not for nothing did has he prosper as a business consultant since his arrival in American 20 years ago.
Grammatical & logically correct for a scenario in which the person is still working now, 20 years after beginning. This contains the very clear implication that the work is still happening.

Either one would be wrong if we used it to describe the other scenario.

The different can be subtle.
5) I saw the movie The Godfather.
6) I have seen the movie The Godfather.
Both are grammatically correct, but they have different implications. Obviously, the action of seeing the movie occurred in the past in both versions. What's different is that the person who says #5 is "done" with that movie. She saw it, experienced it, and has no further need of it. That statement implies zero curiosity about the movie--it's old hat. By contrast, person #6 watched the movie but then the movie in some way remained an influence in her life. In some way, this movie continues to be a kind of "presence" in her life, and if we had to guess, she probably would be interested in seeing it again. These are all subtle emotional implications: the GMAT avoids touchy-feely topics, so you wouldn't have to worry about this for the test.

For more, see:
GMAT Verb Tenses: The Perfect Tenses

Does all this make sense?
Mike