when to use whom and when to use who.

The use of whom and who can be very dificult so,i was looking in the internet for an explanation and this one was the best i found.

Rule. Use this he/him method to decide whether who or whom is correct:

he = who
him = whom

Examples:
Who/Whom wrote the letter?
He wrote the letter. Therefore, who is correct.

Who/Whom should I vote for?
Should I vote for him? Therefore, whom is correct.

We all know who/whom pulled that prank.
This sentence contains two clauses: we all know and who/whom pulled that prank. We are interested in the second clause because it contains the who/whom. He pulled that prank. Therefore, who is correct.

We wondered who/whom the book was about.
This sentence contains two clauses: we wondered and who/whom the book was about. Again, we are interested in the second clause because it contains thewho/whom. The book was about him. Therefore, whom is correct.

Note: This rule is compromised by an odd infatuation people have with whom—and not for good reasons. At its worst, the use ofwhom becomes a form of one-upmanship some employ to appear sophisticated. The following is an example of the pseudo-sophisticated whom.

Incorrect: a woman whom I think is a genius

In this case whom is not the object of I think. Put I think at the end and witness the folly: a woman whom is a genius, I think.

Correct: a woman who I think is a genius

Learn to spot and avoid this too-common pitfall.