How do I know when to use direct or indirect object pronouns?

Direct object pronouns are used to replace the direct object in the sentence. This is the object which has a verb done to it. For example: Doy el libro a Juan (I give the book to Juan). The book is the object, it is being given. If you wanted to avoid saying "book", and say "I give it to Juan", you would use the direct object pronoun: Lo doy a Juan. The indirect object is often a person; it is something which is affected by the verb indirectly. In the example sentence that would be Juan. If you wanted to say "I give him the book", you would use the inderect object pronoun: Le doy el libro. Tip: If you would need to use the pronoun "a" in Spanish, then you use the indirect object pronoun.