I completely agree with Alejandra. Howeeeever I'd like to add something! Even though the correct direct object pronoun is "lo, la, los, las", when in a sentence "lo" is referring to a PERSON (male, of course), it's acceptable to use "le" instead of "lo". That's the only exception to the rule (all the other cases are incorrect and are considered "leísmo", as Alejandra said). So if you're talking about a man, it's correct to say -Ayer lo vi en una discoteca (yesterday I saw him in a disco) -Ayer le vi en una discoteca But if you're talking about a monument, for example, you can only say -Ayer fui a verlo (yesterday I went to see it). I hope it makes sense!

To make it simple, it should always be "lo, la, los, las" when it is a direct object, and "le, les" when it is indirect. E.g: - Yo escribo una carta - Yo la escribo (Direct object) - Tu beberás un vaso de agua - Tú lo beberás (Direct object) - Yo escribo [una carta] a mi hermana - Yo le escribo (Indirect object) (A mi hermano - le) But then... There are tendencies in Spain called "laísmo", "loísmo" and "leísmo". That means in some areas people always use "la" or "lo" instead of "le" (they would say "la escribo", meaning "I write a letter to my sister") and in some areas people always use "le" instead of "la" and "lo" (they would say "le bebo" instead of "lo bebo" meaning "I drink a glass of water") So expect to find some weird sentences out there when you talk to Spanish people or read colloquial texts, but the actual rule is the one I told you first: replace direct objects by "lo" (masculine) or "la" (feminine) and indirect objects by "le" (masculine and feminine).

MandaLE Una manzana. Objeto indirecto. El Perro LO mordio. Objeto directo.