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Anatomy of Spanish: An indirect object [objeto indirecto] is typically a person, animate object, or personified object that receives the result of the action. While a direct object is the thing being acted upon, the indirect object is the person or thing that is the recipient of the action.

In the sentence te mando la carta, it is la carta that is the direct object - being acted upon by the subject through the verb. And the te “to you” indicates who the recipient of the action is; “to whom” or “for whom” an action is done. In a case system, an indirect object is typically identified under the “dative” case.

The indirect object pronouns - me, te, le, les, nos, os - are pretty standard and look similar to the direct object pronouns (which are me, te, lo/la, los/las, nos, os).

For an indirect object, you can make it emphatic by adding an a plus the subject. With me, te, nos, os - who create no confusion as they only apply to yo, tú, nosotros/nosotras, vosotros/vosotras - the addition of the a is even more emphatic. So.. no me digas “don’t tell me” is even more emphatic as no me digas a mí “don’t tell ME”.

It is third person singular (le) and third person plural (les) that are the ones to watch for. Because le mando la carta could be “I am sending the letter to him/her/You”. You can mark the subject’s name or the pronoun; le mando la carta a Ana / le mando la carta a ella… le mando la carta a Luis / le mando la carta a él… and le mando la carta a usted.

The same applies for les which is plural.

When joining a direct object and an indirect object, there’s typically no problem with me, te, nos, os staying the same; te lo mando, me lo mandan, nos la mandas, etc.

With third person - le and les - they change to se when joined with a direct object. This is because le lo or les lo would sound silly and trip up the tongue.

A sentence like… se lo mando, se la mando could potentially refer to ANY third person subject. In proper context, there’s no confusion. But if you think there could be, you can say… se lo mando (a él / ella / usted / ellos / ellas / ustedes). 

Many verbs, especially the verbs like gustar - gustar, encantar, molestar, interesar, importar, fascinar, aburrir, cansar, agradecer etc. - use indirect objects, so it’s important to know how they work and why.

Keep going!