Who vs. whom

Background
The words who and whom are pronouns frequently confused, even by those that are native speakers of the English language. However, there are some simple rules that make it very clear which is appropriate to use. If you can identify the subject and the object of the sentence, you have an excellent start.

When to use "who"
The function of a pronoun is to replace a noun. The function of the pronoun who is to replace the subject of the clause. So, in the sentence "Maria is reading a book." Maria is our subject. To ask the question, we ask "Who is reading a book?"

When to use "whom"
The function of the pronoun whom is to replace the object of the clause. If my sentence were "John invited Maria to the dance." Maria is now the object. We'll ask "Whom did John invite to the dance?"

Be careful of inverted sentence order. Consider these two sentences: "Maria is reading a book." "versus" "A book is being read by Maria." For both sentences, it's okay to ask "Who is reading a book?" Confused? Ask yourself this...who's performing the action here (the book, or Maria)?