In the sentence the action of the verb falls on the object. We say the direct object without any preposition. ex.: Tomo el cafe' - I take the coffee Leo el libro - I read the book there is no preposition before "cafe" and "libro" The opposite , we say the indirect object with some preposition ex.: Hablo a mi hermana - I talk to my sister Trabajo con papa' _ I work with my dad Pienso en ti - I think about you There is preposition before the object. It could be : "a , con , para , por , en , de". When the indirect object is with preposition "a" we can substitute it with pronouns indirect object. The other prepositions could not be substituted with pronouns. Hablo a mi hermana. Le hablo - I talk to my sister . I talk to her.