Who and whoever are used as subjects; whom and whomever are used as objects. These tricky pronouns must be checked in the clause where they belong.

A simple way to check is to ignore everything before who / whom, then focus only on the words that follow it. If needed, rephrase the clause and test it by swapping in he/him, she/her, or they/them.

Quick Rule:

• Use he/she to test for who / whoever (subject).
• Use him/her to test for whom / whomever (object).

For example:

She will hire whoever / whomever can start immediately.

Ignore the first part: whoever / whomever can start immediately.
Test with he/him: he can start immediately — not him can start.
So, the subject form whoever is correct.

More examples:

• I wonder who / whom left their bag on the bus.
  (Cut the first part: who / whom left their bag → he left is correct, so use who.)
• They will reward whoever / whomever they believe worked the hardest.
  (Ignore the parenthetical they believe: he worked the hardest → so whoever is correct.)
• The student who / whom you recommended has been accepted.
  (Rephrase: you recommended him — him is an object, so use whom.)