grammar.pronouns.double-object-se
Se lo, se la: le + lo becomes se lo
When le/les meets lo/la/los/las, the indirect pronoun turns into se: se lo dije, se las mandé.
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Explication
When an indirect object pronoun (le or les) is immediately followed by a direct object pronoun (lo, la, los, las), Spanish replaces le/les with se. So le + lo never occurs; it becomes se lo: ¿Le diste el libro a Ana? — Sí, se lo di.
The order is fixed — indirect before direct — and the pair moves or attaches as a unit: se lo presté, se la mandé, se los enviamos; with commands and infinitives they glue on: dáselo, quiero dárselo. This se is purely a phonetic dodge; it is not the reflexive se.
Because se covers le and les (him, her, you-formal, them) all at once, it can be ambiguous, so Spanish often adds a clarifying a él / a ella / a usted / a ellos: se lo expliqué a ella. The tell-tale sign that you need this rule is two l-pronouns landing next to each other.
Exemples
The report? I sent it to him/her yesterday.
Région: global
I returned them to your parents.
Région: global
The photos? I sent them to your parents last night.