When do we use whom

'Whom' is a relative or interrogative pronoun, Nitharsan, the objective form of 'who'.

Use 'whom' as an object of a preposition:

'With whom are you attending the concert?'
'May I ask to whom I am speaking?'
'A general is the military officer under whom all other ranks serve.'

Or, use whom as the object of a verb:

'Whom did you invite to the wedding?'
'John Kerry is the candidate whom George W. Bush defeated in the last election.
'Survival of the fittest is mostly a case of who eats whom.'