Who and Whom.
This seems to cause some problems but can be easily sorted out.
Essentially, who is a subject pronoun and should be used to replace the subject of the verb.
Whom is an object pronoun and should be used either to replace the object of a verb, or to follow a preposition.
A simple test is to see whether you can replace the who/whom with a subject pronoun (I or he) or an object pronoun (me or him).
Whom did you see? (Did you see him?)
I cannot see who is in the classroom. (He is in the classroom.)
To whom it may concern. (To him...)
Who has lost their shoes? (He has lost his shoes.)
Test your understanding of who and whom using this exercise.