What about "por _______"?

Question

I just took one of the B1 quizes and I got this one wrong:

La había comprado para sus padres. ________ la había comprado. (He'd bought it for his parents. He'd bought it for them.)

It says the answer is "Se la habia comprado." I put "Por ellos, la había comprado."

Is it ever correct to "por" whoever, instead of "se"?

It sounds more natural to me to say "La había comprado por ellos," rather than "Se lo había comprado." It also seems clearer because "ellos" is specific (them), where as "se" could be you, him, them...

Is that wrong? Less common? Common only in certain countries or situations?

Silvia Piriz · Accepted Answer

Hola Elizabeth C.
This is a great question, and it's completely normal to find this topic tricky.
In the sentence "La había comprado para sus padres. → Se la había comprado", the word "se" replaces "les" (the indirect object pronoun for "them") because when combined with the direct object pronoun "la", Spanish grammar avoids the awkward repetition of "les la" and changes it to "se la". This is a standard and necessary change.
Now, regarding your suggestion—“Por ellos, la había comprado”—this is grammatically correct, but it changes the meaning. Using "por ellos" implies he bought it because of them or on their behalf, rather than for them in the sense of giving them the item. So while your version isn’t wrong, it doesn't mean the same as using "se", which clearly marks the action as being directed toward someone (i.e., to/for them).
Also, while “se” might seem ambiguous, this is very natural in Spanish and usually resolved through context. You're absolutely right that “ellos” feels more specific, but Spanish often relies on prior information to clarify "se".
Keep going—you’re asking exactly the kind of detailed questions that lead to real fluency!
Saludos
Silvia