Learn the rule (or how to avoid the issue).
“Whoever” Versus “Whomever”
If you’re comfortable with everything so far, way to go! You probably know more about the use of “whom” than the majority of everyday English speakers. However, they say a little bit of knowledge is a dangerous thing, and you now know enough to get into trouble. You must continue your Grammar Jedi training, or risk being pulled to the Dark Side, or at least, the Muddled and Confused Side. Waiting to trap the unsuspecting writer is the issue of “whoever” versus “whomever.”
When you’re faced with a “whoever/whomever” choice, it’s often a good time to dodge the issue and simplify your sentence.
At first, there might seem to be no problem. Take a clause like “whoever did this.” We use “whoever” because it’s the subject of “did.” Now take a clause like “whomever I hire.” We use “whomever” because it’s the object of “hire.” So far, so good.
The trouble begins with the fact that “whoever did this” and “whomever I hire” aren’t just ordinary clauses; they’re noun clauses—clauses that act just like nouns. When you use these noun clauses as subjects or objects in larger sentences, the picture gets more complicated. Suppose the noun clause “whoever did this” is part of a sentence like “I want to speak to whoever did this.” Uh-oh, you may think, “whoever” is the object of “to,” and it should be “whomever”!