Questions to find the indirect object

Question

"The indirect object answers the question “To whom?” or “For whom?” the action of the verb is performed." For example:

1. He gives María the book.
   To whom does he give the book?
   To María.
   IO = María
2. He buys me flowers.
   For whom does he buy the flowers?
   For me.
   IO = me

I would like to know why in sentence number 1 we ask to whom and not for whom, and why in sentence number 2 we ask for whom and not to whom. In other words, how do I know when to ask to whom and when to ask for whom in order to find the indirect object.

Answer 1

We give something to someone. To whom did he give the book?

We buy a gift for someone. For whom did he buy the flowers?

Who are those flowers for?
--They're for my girlfriend.