How to use 'who' and 'whom'

Students of English often get confused when it comes to the usage of 'who' and 'whom' especially the latter.

'Who' is used to ask the person as a subject in a question. For example, you can say, 'Who is the boy standing there?' When I ask this question, I want to know the name of the boy or his relation with the person I ask. He can say, "He is Albert." or "He is my cousin."

As for 'whom', it is used to ask the object in a question. For instance, you can ask, "Whom did you see just now?" I would like to know the person you saw when I put forward such a question.

'Whom' is also used after a preposition. Here are some examples:
1 To whom did you give my book?
2 From whom did you get this doll?
3 With whom did you play just now?