“Exchange” means trade or swap: you give them something and they give you something back for it. It’s not a gift, its one thing traded for another: I give you the book and you give me a scarf. So maybe that’s why it needs a different word from the word with a gift.

I don’t know the Spanish rules at all, but I can see that you’re misusing “exchange.”

Broadly speaking, prepositions establish a relationship between two nouns. Both para and por are prepositions. Prepositions are some of the most difficult structures to master in any language because they do not follow strict rules and have several meanings and uses.

Another problem with prepositions is that of usage; i.e. when you use a certain preposition in one language to establish a relationship between two nouns, another language will use a different preposition to establish the same relationship. Such is this case you are talking about.

When you say that it makes no sense, you are trying to force a logic that does not apply to prepositions. Usage is what matters the most when dealing with prepositions. How people of a certain language use prepositions is what ultimately determines the preposition you need to use. It is a waste of time to try to take the logical route when you are dealing with prepositions.

Para can be translated as for or to in English depending on the case.
Por can also be translated as for, but what matters is not only the meaning and translation, but also the usage of the words.

In Tengo un libro para ti, the word para means for . It translates I have a book for you.

Tengo un libro por ti sounds awkward to a Spanish native speaker because even though technically por can be translated as for, in this case, Spanish speakers would not use por but para.

My mom taught me that when you are thanking someone for something you always say gracias por. Your mom is right. For example, Gracias por el libro.

But, it doesn’t make sense for the reasons you are giving: That makes sense because someone exchanged something to you by giving you something and you are thanking them for exchanging something to you. If I give you a gift or if I return a book I borrowed from you, you can say Gracias por el libro., but we are not exchanging anything.

It makes sense because of usage, por is the word we use to thank someone for something. But when we give someone a gift (not an exchange) we say _Gracias por…”

You say that Para actually translates to in order to. Like this: Necesito un lapiz para hacer mi tarea.

In this case, para actually translates as to. So, your sentence translates as I need a pencil to do my homework.

I hope I was able to help you understand that there isn’t that much logic in the use of prepositions, it is actually very arbitrary.

I do not know what level is your class. For basic and intermediate students, follow usage and memorize situations and phrases. For more advanced students there are other considerations that require a deeper semantic analysis as to why, when and how to use prepositions.

Orale’ chula! The correct sense depends on context – But also other stuff, including maybe whether you’re talking to a Euro Spanish speaker or Mexican.

But I’m a gringa – So this is really a question for bob.

Bob? Oh BOB…

Prepositions can be tricky like that, as @Yeahright says. In short, “Tengo un libro por ti” would be translated as “I have a book by you”, but even then, it does sound a bit akward.

@JLoon You keep saying my name, though ~

@bob_ Mi querido Watson, a bit awkward is an understatement.

Tengo un libro por ti or I have a book by you makes no sense in either Spanish or English. You would have to ask the person to clarify what they mean.

You can assume that the person means a book written by you, but you would still have to ask them if that is what they meant.

Oh Roberto ;p

Oh gosh. I think I ask my husband every other year a por vs. para question for the last 30 years.

Prepositions are so difficult. Spanish speakers get them wrong all the time in English, and English speakers get them wrong all the time in Spanish.

Just two weeks ago when I asked him again about por and para, he described por as the word through (a meaning I had not even realized) and also por as exchange as you said, but I think It’s exchange for things like dos dólares por tres chocolates. When you are talking about giving something to a person or taking it for yourself you use para. Un chocolate para ti.

I probably shouldn’t even answer because I have so much trouble with it myself.

I hope the native speakers here correct me if I wrote any incorrect information.

Be grateful your mom corrected you. My husband and MIL let me babble on incorrectly all day long. Not that I want to be corrected every five seconds, but a little here and there for mistakes I repeatedly make would be helpful.

It’s not really an exchange though. You are giving them something. And exchange would be, Te doy 20 dolares por ese libro. You are giving money in exchange for the book.