pronouns - What’s the rule for using “who” and “whom” correctly?

I can never figure out whether I should use who and whom. Most people use who for both colloquially, but some people say this is not correct.

What’s the rule for using who and whom correctly?

Answer

The easy way to tell which is technically correct is to substitute he and him for who and whom, then rearrange the word order to see which sounds right.

“Who were you speaking to?” becomes “You were speaking to he” — which is clearly incorrect.