How do you know when to use "who" or "whom" in a sentence?

How do you know when to use "who" or "whom" in a sentence?
if you can replace a word with "he" or "she," then it is the subject of the sentence and you should use "who." If you can replace the word with "him" or "her," use whom.
Example: Who or whom ate the cake.

He ate the cake........use who

Note
Him ate the cake...so you would not use whom