When there is an indirect pronoun in a sentence we use: me, te, le, nos, os, les. However when the indirect pronouns le and les (meaning to him / to her / to them) are accompanied by the direct pronoun lo, la, los or las (meaning it / them) then le and les become se to avoid a clumsy sounding le lo, le la, les los etc, which is tricky to pronounce. Listen to this example:
Laura se lo compró.Laura bought it for her. Remember that no matter how many different pronouns are used in the sentence, the indirect one comes first and the direct one comes after.
Listen to these examples:
No se lo dije.I didn't say it to her.
Dáselo a ella.Give it to her.
Se la dimos.We gave it to them.
Quiero comprárselos.I want to buy them for them.
María está comprándoselos.María is buying them for him.
No sabemos por qué se los ofrecieron.We don't know why they offered them to them.
Note that se has a different position depending on the verb form.
With a conjugated verb, se is separate to the verb and in front.
With an infinitive, imperative or gerund, se comes straight after the verb form as part of the same word.
Quiero comprárselos. (infinitive)
Cómpraselos. (imperative)
Estoy comprándoselos. (gerund)
ATTENTION: When the gerund or infinitive are acompanied by a conjugated verb it is also correct to place the se in front of the conjugated verb, but never between the conjugated verb and the gerund or infinitive. Here are some examples to show the two possibilities:
No se lo puedo apretar.I cannot tighten it for her.
No puedo apretárselo.I cannot tighten it for her.
Estamos decorándosela.We are decorating it for him.
Se la estamos decorando.We are decorating it for him.
Here are some more examples:
Las zapatillas pónselas al lado de la cama.The slippers, put them next to the bed for her.
No se los pagues, él tiene bastante dinero. Don't pay those for him, he has enough money.
¿Por qué no se lo comunicaste antes?Why didn't you let them know about it before?
Se lo daré cuando la vea.I will give it to her when I see her.
El regalo, yo se lo dejé ahí.The present, I left it there for him.
¿Crees que se lo habrá dicho ya?Do you think she will have told him yet?
Pregúntaselo ahora mismo. Ask her about it right now.
Estoy montándoselas en el salón.I am putting them up for him in the living room.
See also:
Using lo, la, los, las = him, her, it, them (direct object pronouns)
Using le and les = [to] it,him, her, them (indirect object pronouns)
Quiero comprárselos.I want to buy them for them. Se la estamos decorando.We are decorating it for him. No se los pagues, él tiene bastante dinero. Don't pay those for him, he has enough money. Estamos decorándosela.We are decorating it for him. No sabemos por qué se los ofrecieron.We don't know why they offered them to them. ¿Crees que se lo habrá dicho ya?Do you think she will have told him yet? Laura se lo compró.Laura bought it for her. Dáselo a ella.Give it to her. No puedo apretárselo.I cannot tighten it for her. Las zapatillas pónselas al lado de la cama.The slippers, put them next to the bed for her. Estoy montándoselas en el salón.I am putting them up for him in the living room. María está comprándoselos.María is buying them for him. Se la dimos.We gave it to them. Pregúntaselo ahora mismo. Ask her about it right now. No se lo dije.I didn't say it to her. El regalo, yo se lo dejé ahí.The present, I left it there for him. No se lo puedo apretar.I cannot tighten it for her. Se lo daré cuando la vea.I will give it to her when I see her. ¿Por qué no se lo comunicaste antes?Why didn't you let them know about it before? 
