Double Object Pronouns refer to the usage of Direct Object Pronouns and Indirect Object Pronouns with the same verb.
In addition, it is important to note that when this happens, the indirect object pronoun must always be written first.
Now, the Direct object pronouns are:
1st person singular: Yo me
2nd person singular (informal): Tú te
2nd person singular (formal): Usted lo/la
3rd person singular: El/eso lo, Ella/esa la
1st person plural: Nosotros nos
2nd person plural: Ustedes(In latinAmerica) los/las/Vosotros(In Spain) os
3rd person plural: Ellos/Ellas los/las
And the Indirect object pronouns are:
1st person singular Yo: me
2nd person singular (informal) Tú: te
2nd person singular (formal) Usted: le
3rd person singular Él/Ella/eso/esa: le
1st person plural Nosotros: nos
2nd person plural Ustedes (In latinAmerica): les
2nd person plural Vosotros (In Spain): os
3rd person plural Ellos/Ellas: les
Knowing this, let's begin with the answers:
1. Mi tío me los compró.
Zapatos (shoes) directly receives the action of the verb compró (he bought), hence is the direct object. Zapatos is a plural masculine noun, related to the 3rd person plural ellos (they), and the corresponding direct object pronoun is los.
On the other hand, yo (I) indirectly receives the action of the same verb. Yo (I) is the 1st person singular, therefore the corresponding indirect object pronoun is me.
Remembering the rule: the indirect object pronoun must always be written first, the complete sentence is:
Mi tío me los compró.
2. Se las escribí a ellos
Postales (postcards) directly receives the action of the verb escribiste (you wrote), hence is the direct object. Postales is a plural feminine noun, related to the 3rd person plural ellas (they), and the corresponding direct object pronoun is las.
On the other hand, ellos (they) indirectly receives the action of the same verb. Ellos (they) is the 3rd person plural, therefore the corresponding indirect object pronoun is les.
Now, according to spanish (castillian) grammar we can use se as an indirect object, in order to substitute the indirect object le or les.
This se is immediately followed by one of the following pronouns depending on the direct object (the part of the sentence that directly receives the action):
lo (singular masculine)
la (singular femenine)
los (plural masculine)
las (plural femenine)
This means that in this case les can be substituted by se las
Remembering the rule: the indirect object pronoun must always be written first, the complete sentence is:
Se las escribí a ellos
3. Su tío se lo recomendó
Plato (dish) directly receives the action of the verb recomendó (recommended), hence is the direct object. Plato is a singular masculine noun, related to the 3rd person singular it, and the corresponding direct object pronoun is lo.
On the other hand, he/she, which is the 3rd person singular indirectly receives the action of the same verb, therefore the corresponding indirect object pronoun is le.
Again, we can use se as an indirect object, in order to substitute the indirect object le or les.
This se is immediately followed by one of the following pronouns depending on the direct object (which is lo):
This means le can be substituted by se lo
Remembering the rule: the indirect object pronoun must always be written first, the complete sentence is:
Su tío se lo recomendó
4. Sonia nos la va a abrir
In this sentence puerta (door) directly receives the action of the verb abrir (open), hence is the direct object. Puerta is a singular feminine noun, related to the 3rd person plural it, and the corresponding direct object pronoun is la.
On the other hand, nosotros (we) indirectly receives the action of the same verb. Nosotros (we) is the 1st person plural, therefore the corresponding indirect object pronoun is nos.
Remembering the rule: the indirect object pronoun must always be written first, the complete sentence is:
Sonia nos la va a abrir