Answer: Yo se lo compro
This is a good example of the usage of se as an indirect complement, in order to substitute the indirect complement le or les.
What does this mean?
Let’s begin by explaining that the Indirect Complement of a sentence is the person, animal or thing that indirectly receives the action of the verb.
To identify it in a sentence, we must ask to the verb ¿a quién? (to whom?) Or ¿para quién? (for whom?)
In the case of the sentence Yo compro el carro para Pablo (I buy the car for Pablo) if we ask ourselves the following question:
¿para quién compro el carro?
(For whom do I buy the car for?)
The answer is:
Para Pablo
(For Pablo)
Being Pablo who indirectly receives the action, therefore the indirect complement of the sentence.
Now, the Indirect Object Pronouns replace a word or phrase, which in the sentence fulfills the function of indirect object and they are written before the verb, when it is conjugated.
These pronouns are listed below:
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
Then, returning to the sentence Yo compro el carro para Pablo, knowing that the indirect object is Pablo, who is the 3rd person singular Él (he); the indirect object pronoun in Spanish is le.
In that way the initial sentence changes to:
Yo le compro el carro (a Pablo)
I bought the car (for Pablo)
This is where the personal pronoun se comes in, as an Indirect Complement, and replacing le or les (indirect object pronouns), 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)
In this case lo is used because Pablo (he) is masculine and singular, therefore the sentence:
Yo compro el carro para Pablo
Changes to:
Yo se lo compro
Where se and lo are the two object pronouns.