1. A pronoun must agree with its antecedent in person, number and gender.
Ex- Every student must bring his luggage.
All students must do their homework.
Each of the boys must carry his own bag.
2. The pronoun ‘one’ must be followed by ‘one’s’.
One must finish his homework in time. (Incorrect)
One must finish one’s homework in time. (Correct)
3. Whom is an objective pronoun, which is a pronoun that receives the action of a verb. It also has two main uses:
It is the object of a verb—e.g., Whom should I call?
It is the object of a preposition—e.g., From whom did you get this information?