Commonly Confused Words (3): Who vs. Whom
Who functions either as the subject of a sentence ("Who told Jason that?") or as nominative pronoun in the predicate ("I met the one who was told by Marie.")
Whom functions as the object of either a verb ("Marie saw whom?") or a preposition, such as for whom, by whom, to whom, ... ("Jason talked to whom?").
Generally, if you can replace the word with he or she, use who ("He/Who told Jason that?").
If you can replace it with him and her, use whom ("Marie saw him/whom?")
In some cases, you may need to restructure the sentence for this to work: "Who told Jason that?" — "She told Jason that." ✓
"Whom did Jason tell?" — "Him did Jason tell." ✗ → "Jason told him." ✓
"Why does whom sound weird in the beginning of the sentence?"
The usual structure of an English sentence is Subject, Verb, Object. Putting "whom" (the object) in the beginning can sound off. To avoid this, you can restructure the sentence: "Whom did Jason talk to?" → "Jason talked to whom?"
End Note: Though it is not grammatically correct, it is generally not considered a big mistake to use who when whom should be used, especially in casual speech. Sometimes, it may even be preferred as it sounds more natural, even though it is technically incorrect: "Who did Jason tell?"