First of all, 'Who' is a subjective case for the subjective pronoun 'he', and is used as a relative pronoun in the middle of the sentence. 'Whom', although is also a relative pronoun, is an objective case of the pronoun 'he' and is also used as a relative pronoun in the middle of the sentence. Not only for the complex sentences, where both these words are used in the middle of the sentences to relate the other part of the sentence to make it one, these two pronouns are also used in the Interrogative sentences but at the beginning instead of in the middle of the sentence. 'Who' is used as the subject of the verb in the sentence e.g. 'Teacher punishes the student who came late', 'I played with the student who challenged me'. For interrogative sentence, 'Who broke the glass'? 'Who is there behind the wall?', 'Who fought with you last night'? On the other hand, 'Whom' when comes as a related pronoun in the middle, refers to Object, playing the role of its pronoun e.g. 'She returned the pen to the boy whom she borrowed earlier', 'She is the girl whom our teacher called diligent student'. For the interrogative sentence e.g. 'Whom did you hand over the letter?' 'Whom did she call for help'?
Both these words are pronouns but with the difference that one is used as a Subject and other is used as an Object in a sentence
The pronoun 'Who' is placed in the middle of the sentence to provide detail about the subject; 'Whom', also provides detail, but unlike 'who' for the Object in a sentence
Basically, 'Who' is used as a Subject in a sentence; 'Whom', is used at the opposite side of 'Who' as Object in a sentence
'Whom' can never be used as a Subject; 'Who' is always used as a Subject and can never come as Object
Who
(interrogative) What person or people; which person or people; asks for the identity of someone; used in a direct or indirect question.
Who is that? (direct question)
I don't know who it is. (indirect question)
Whom
(interrogative) What person or people; which person or people.
Who
(relative) Introduces a relative clause having a human antecedent.
Whom
As the object of a verb.
Whom did you ask?
Who
With antecedent as subject.
That's the man who works at the newsagent. (defining)
My sister, who works in the accounts department, just got promoted to manager. (non-defining)
Whom
As the object of a preposition.
To whom are you referring?
With whom were you talking?
Who
(non-formal) With antecedent as object: whom.
Whom
(relative) Used to refer to a previously mentioned person or people.
That is the woman whom I spoke to earlier. (defining)
Mr Smith, whom we all know well, will be giving the speech. (non-defining)
He's a person with whom I work. (defining)
We have ten employees, half of whom are carpenters. (non-defining)
Who
Whoever, he who, they who.
Who insults my mother insults me.
Whom
The person(s) whom; whomever.
To whom it may concern, all business of John Smith Ltd. has now been transferred to Floggitt & Runne.
Who
A person under discussion; a question of which person.
Whom
The objective case of who. See Who.
And every grass that groweth upon rootShe shall eke know, and whom it will do boot.
Who
Whose
Who phone just rang?
Who
One; any; one.
As who should say, it were a very dangerous matter if a man in any point should be found wiser than his forefathers were.
Who
A United Nations agency to coordinate international health activities and to help governments improve health services
‘Who’ is a Relative Pronoun in the English grammar to play the role of a ‘connector’ in the middle of the sentences where it is needed to refer to the subject mentioned earlier in the sentence. This relative pronoun relates two parts of the sentences to make it as a whole complex sentence. Not only as a relative pronoun, ‘Who’ is also put at the start of the sentence, but in such situation where to seek some information in the interrogative sentences. ‘Who’ can only be used as the subject of the verb and that it can never be employed as an Object of the verb in the sentence.
‘Whom’ is another relative pronoun in the English grammar which is also used to related two parts of a sentence. However, unlike ‘Who’, ‘Whom’ can only be used as the object of the verb instead of Subject in the sentence. It can either be used as ‘Direct Object’ or ‘Indirect Object’ in a sentence, not only for Affirmative sentences but also for the Interrogative ones. For instance, as a direct object, ‘Whom are you following in life’? as an indirect object, ‘To whom was the dog running after’?
