Choosing whether to use "who," "whom," or "whomever" can be tricky if you do not understand the difference between subject and object pronouns.
Subject pronouns: I, he, she, it, you, we, they
A subject is the main noun doing the action in a sentence.
Object pronouns: me, you, him, her, them, us, it
An object is the noun that receives the action.
Ex. I saw a girl whom I'd thought went to the same middle school.
It should be "who" instead of whom, because the
verb went needs a subject. Only
who can be a
subject.
* A simple trick to determine which one to use is to substitute the "who" or "whom" part with a different pronoun. Only one will sound obviously correct:
Ex. We all know he slapped her. ✅ → We all know him slapped her. 🆇
The same goes for "
whomever" (
object form) vs "
whoever" (
subject form).
Ex. I will play with whomever finishes their homework first.
It should be "whoever," because the object is the entire clause "whoever finishes their homework first" is the
object. The
verb finishes needs a subject, which would be whoever.
A final tip: "whom" is considered mostly fit for formal situations, so if you think it isn't necessary, then it isn't!