If you can replace the word with “he”, you always use who. If you can replace the word with “him”, you can use whom if you want to.

Whom did you lead into battle?

I led him into battle.

Who ate all the cake?

He ate all the cake.

The key takeaway is you can always use who and it will be correct, because who is both a subject and an object. So, if you don’t want to bother with the rule, just stick to who and you can’t go wrong.

if you are familiar with object vs subject in grammar you already know the rule, who is used when it’s the subject, whom when the object:

Who is that?

That’s who ate my ice cream.

Whom did you give ice cream to?

The ice cream went to the one whom I saw first.

This rule is the same as knowing when to use she or he vs when to use her or him, it’s no different.

However, most people don’t use whom correctly and it can just be avoided entirely, most people will just use who as the object anyway and it will sound more natural to them:

Who did you give ice cream to?

The ice cream went to the one who I saw first.

Using whom in these cases can make you sound formal or fancy, and draws attention.

It’s pretty much a dead language feature anyway, at least in my area. Whom sounds pretentious as hell if you actually say it. Like, you’d get away about as well with thee or thou.

To whom/for whom is supposed to be the rule for when to use whom, but in American English it sounds way too formal.

There’s a pretty trivial rule for getting this right. Phrase your sentence using who/whom as a question. Respond with he/him. If your response contains a “he”, your initial statement should be “who”; if it contains a “him” then you’re looking at a “whom” use.

• ex: “To who/whom should the gold be given?” -> “To him” -> “whom”
• ex: “Who/whom wants the gold?” -> “He wants the gold” -> “who”
• ex: “Who/whom did you see at the party?” -> “I saw him” -> “whom”
• ex: “The man who/whom called earlier is here” -> “Who/whom called?” -> “he called” -> “who”