- When you speak to him remember to whom you are speaking.
- When you speak to him remember who you are speaking to.
- When you speak to him remember whom you are speaking to.
What is the difference among these three? If any.
To whom should I send the letter?
Send it to whom?
Whom should we send?
"Whom" and "to whom" are the same thing. The "to" is just the preposition included with the pronoun.
We know who has the answer.
In fact, you are unlikely to hear/read "whom" outside of formal writing.
All 3 of them mean the same thing. They differ only in formality, correctness of grammar, and evident education level of the speaker (or writer).
When you speak to him remember to whom you are speaking.
follows all the rules of English grammar and is consequently the most formal and educated-sounding.
When you speak to him remember whom you are speaking to.
has the correct pronoun case (whom), but breaks a lesser (?) grammar rule by separating "to" from "whom" and sticking the preposition at the end of the sentence. Some people claim this is an error, but I have heard others disagree. In any case, this sentence sounds more casual and slightly less "professorial".
When you speak to him remember who you are speaking to.
is the one I would be most used to hearing in every day (US) English. Strictly speaking, I guess it is incorrect, as it uses "who" (nominative case) as the object of the preposition "to", but it is very commonly spoken and written, and it sounds normal, idiomatic, and informal.