I think that for some people learning English, knowing when to use "to" and "for" can be an issue. It is probably even a bigger issue for people who speak Italian, Spanish or another language where the same indirect object pronoun is used to indicate both concepts, especially since some English verbs take "for" and some take "to" and there is no one-to-one mapping onto the correct English preposition. For instance,
In Italian:
1. Gli ho comprato un caffè.
I bought a coffee for him. / I bought him a coffee.
* gli = for him
2. Gli ho cucinato il pollo.
I cooked chicken for him. / I cooked him chicken.
* gli = for him
BUT...
3. Gli ho inviato il libro.
I sent the book to him. / I sent him the book.
* gli = to him
4. Gli ho portato il libro.
I brought the book to him. / I brought him the book.
* gli = to him
As you can see, "gli" maps to "for him" or "to him" depending on the English verb. It doesn't make sense to say "to cook chicken TO HIM". It does make sense to say "I brought the book for him." but this means something different than sentence #4. It means that he couldn't bring the book himself so you brought it instead, or that you brought the book so that he could use it. So to say "I brought the book for him." as the translation of #4 really does not make sense and is incorrect. How confusing, huh? Hehehe.
I think things get even more complicated because we use "for" to mean "on behalf" as well and it can even appear in a sentence with a "to + indirect object" pattern. For instance, the following sentence:
5. Gli ho inviato il libro per lei.
I sent the book to him for her. / I sent him the book for her.
= I sent the book to him on her behalf.
Imagine that your girlfriend wants to send a book to her brother. She has to work and therefore does not have time to go to the post office herself. You decide to be nice and to do her a favor. You take the book to the post office and mail it to her brother. In this sentence, the "for" in "for her" actually means "on her behalf". She can't do it, so you are doing instead because you are doing her a favor. So here, "per lei" maps to "for her" and you shouldn't confuse this "for her" with an indirect object in English. :)
