Any easy trick I use to help remember which to use is to replace who/whom with he/him to see which makes more sense in the context. "He" could not grasp. => "W

Any easy trick I use to help remember which to use is to replace who/whom with he/him to see which makes more sense in the context. "He" could not grasp. => "Who" could not grasp. I should vote for "him" => "Whom" should I vote for?