Who is it? Whom is it? Who knows??

A grammatical subject that I have never quite got the grasp of is when to use who and whom. To really tell you the truth I never use whom ever, but I have seen throughout my English classes that there is a difference. I think I struggled with it because I can never tell the difference. I do not know if i need who or whom when I write. I some cases I can see that it is obvious that I need to use who, but sometimes ill get corrected and they'll say that I need to use whom. Who knows really?

The way I now remember how to tell the difference between who and whom are with these simple definitions:
Whom: is an object
Who: is a subject

Also you can try the he/ him approach.
Whom = him
Who= he

Ex:Original statement: James didn’t know who/whom would win.

Replace with he/him: James didn’t know he would win.

Therefore: James didn’t know who would win.

Original statement: With who/whom should Trey go?

Rephrase and replace with he/him: Should Trey go with him?

Therefore: With whom should Trey go?

For more help these following websites are the places to go:
http://editing.suite101.com/article.cfm/how_to_use_who_and_whom_correctly
http://web.ku.edu/~edit/whom.html