If there’s one grammar issue that seems to confuse almost everyone, it’s when to use WHO and when to use WHOM. Both are pronouns, but I’ve found that using the word “pronoun” in an answer tends to baffle already confused people even further, so here’s a less technical explanation:
If you would use HE or SHE in a similar construction, then use WHO (or WHOEVER):
Gloria has no idea who wrote the blog post. (CORRECT)
Gloria believes that whoever takes the job will have a lot of work to do. (CORRECT)
Gloria hired the person who* she believes will do the best job. (CORRECT)
*This one is tricky because people often think, “Wait! Gloria believes him!” But in this case she does not believe him. She believes that he will do the best job.
If you would use HIM or HER in a similar construction, then use WHOM (or WHOMEVER):
I’m going to hire whomever I want. (CORRECT)
Gloria and Domingo are people whom I want to support. (CORRECT)
Domingo is a man whom I can see going very far in his career. (CORRECT)
I wouldn’t know whom to call in case of an emergency. (CORRECT)
I know how vexing this topic can be, so I hope this post helps unravel the mystery for whoever reads it.