Basically, 'who' is a subject pronoun and 'whom' is an object pronoun. So you use 'who' when the word is replacing the subject of a sentence, and 'whom' when it is replacing the object.Originally Posted by english learner
1) "Mirror, mirror, on the wall - who is the fairest of us all?" - 'Who' is the subject of the sentence here.
2) "Ask not for whom the bell tolls - it tolls for thee..." - As in the title of the Hemingway novel, 'the bell' is the subject, and it tolls for someone - the object of the sentence.
No. 2) is a good example of the rule that in English a dative phrase (to/for followed by a noun or noun phrase), or an ablative phrase (by/with followed by a noun or noun phrase), takes the accusative (object) case of the noun.
John: "I was partnered with Jim at tennis today."
Jane: "You were partnered with WHOM?"
You will find many native speakers flout this rule, and would use 'who' in the above example - in fact, in speech I probably would too . 'Whom' is losing ground in modern English, as it sounds like it belongs with Old English pronouns such as 'thou', 'thee' and 'thy'.