Hola Mónica, Para usar los pronombres de la 3ª persona lo(s), la(s), le(s) debes tener en cuenta dos cosas: 1. La función sintáctica que desempeña el pronombre (Objeto Directo o Indirecto) 2. El género y si es singular/plural de la palabra a la que se refiere Objeto Directo : Singular Plural Masculino Lo/Le Los Femenino La Las Neutro Lo - Objeto Indirecto: Le Les Ejemplos Objeto Directo: ¿Has visto a Juan? Sí, lo vi ayer. (LO se puede sustituir por LE si se refiere a un hombre) ¿Has visto a Juan y a los niños? Sí, los he visto en el parque. Compré la medicina y se la di sin que nadie me viera. ¿Has recogido a las niñas? Sí, las recogí antes de ir al taller. Ejemplos Objeto Indirecto: Le pedí disculpas a mi madre. Le dije a su hermana que viniera. Les di un regalo a los niños. Espero haberte ayudado.

Alberto wrote about when to choose a particular pronoun depending on whether it functions as a direct object or as an indirect object. You also asked about when to use object pronouns before a verb and when to use them after a verb. Verbs function as the central part of the predicate of a sentence, and sometimes they function in another role in the sentence. Sometimes a verb has only one verb form ("leo el libro"), and sometimes it has two or more verb forms ("estoy leyendo el libro", "he leído el libro"). When a verb has only one word, it is one of the approximately 48 distinct forms of the verb, and it may be a non-finite form (infinitive, present participle, past participle) or a finite form (everything else). Finite forms always show person and number of the subject, tense (future, present, past), and mood (indicative, subjunctive, imperative). 1. Indicative or subjunctive mood: object pronouns go in front of the verb as separate words: "Yo (no) doy el libro a María" -> "Yo (no) se lo doy". 2. Positive imperative mood: object pronouns are connected to the end of the verb: "dáselo", "dádselo", "déselo", "dénselo" = "give it to her"; and "démoselo" = "let's give it to her". 3. Negative imperative mood: object pronouns go in front of the verb as separate words: "no se lo des", "no se lo deis", "no se lo dé", "no se lo den" = "don't give it to her"; and "no se lo demos" = "let's not give it to her". 4. With infinitives and present participles: object pronouns are connected to the end of the verb: "dárselo", "dándoselo". 5. As a general rule, it is not possible to use object pronouns with a past participle that is by itself. When a verb has two or more words, usually you can choose to put all pronouns in front of a form in group 1 or connect all pronouns to the end of a form in group 4: "estoy leyendo el libro" -> "lo estoy leyendo" OR "estoy leyéndolo".