Say a huge rock rolled down from a cliff and killed a passenger by the window of a passing-by bus which I was on. HOWEVER, before the rock hit the bus, I had switched seats with the poor man, the victim.
So I can say :
a.It could be me
b.It could have been me
c.It could not have been him
d.It could have not been him
Which is correct and why, please?
All of these are grammatically correct. In the situation you describe, talking about an "twist of fate", the most common expression would be:
That could have been me.
In this case the present perfect expresses a potential life experience, narrowly avoided.
However, the negative of this generally means simple lack of ability or potential, and not fate:
Larry could not have been (or simply could not be) the one who robbed that bank yesterday. He's on vacation in Belize.
Instead you would use a conditional:
If he hadn't switched seats with me, it would not have been him (who died that day).
or, alternately:
If we hadn't switched seats it would have been me (who died that day).
Under all of those possibilities, in correct, "formal" English (English almost never spoken, but grammatically correct), it should read:
"It could have been I."
It would also be "I" instead of "me" for every one of your examples and, in the answer above, it should read:
"It could have been he (not him) who died that day."
This is an example of the "predicate nominative" in English. Most English speakers say things such as "It's me" or "It's him or us", but correct English (formal English used especially in writing) would say "It is I" or "It is he or we". For instance:
"It is I who am to blame."
"It is we who have to live on this planet so it is we who must protect it."
"It is they who are the reason that we have failed."
"It is she whom you want."
Again, this is very formal English; therefore, I am teaching you the English that you would use when you are writing a formal essay, paper, dissertation, treatise, etc. in English. This would almost never be heard in Modern English conversation, however, wherein the "disjunctive pronoun", which is in the oblique case, is always used in colloquial speech. Sigh, it is not I who write the rules. (Notice the subject-verb agreement between "I" and "write".)
