Why is it "no te los compres" and not "no se los compres" ?
Indirect object pronoun question.
It's my understanding that when you have a direct object pronoun and an indirect object pronoun together that the indirect object pronoun changes to "se"
Have I misunderstood the rule, or is this an exception? If so, what's the cause of the exception, is it because it's a negative imperative?
Thanks!
5 Answers
It's my understanding that when you have a direct object pronoun and an indirect object pronoun together that the indirect object pronoun changes to "se"
No. Only when the indirect one is "le" or "les".
This "te" is very colloquial, and poorly documented in most textbooks and grammars (often, not even mentioned). You can omit this particular pronoun in theory, but it puts a lot of stress on the fact that if you buy it, it will be yours, you'll own it. If you omit it, one can only guess who are you buying it for. In spoken Spanish is very common, because people tend to be very expressive in order to make communication more effective.
Well, Im a spanish major, and its because if you said "no se los compres" the verbs dont match up and the sentence doesnt make sense, the "se" doesnt refer to anyone in this sentence, it just doesnt make sense. And also, its a command, so therefore you specify who is being commanded, which is "you" in this case. Almost always youre going to see things like "no te los compres" or "No se los compre", just to verify the verb.
Hard to explain, more one of those things you just learn and it kinda just makes sense after a while and you just use it without thinking. 
On the other end, seeing the second comment, that is true, "se" can refer to people, but the way this sentence looks, it looks more like a command to that one person, telling them not to buy them. but It just depends on the rest of the conversation to really know why its not "se" instead of "te"
I cannot help you with the DO or IO but let's see if this makes sense to you...
No te los compres = Do not buy them
No se los compres = Do not buy them from him/her/them
or
No se los compres = Do not buy them to/for him/her/them
Both these sentences are correct. They are commands. (YOU do this or that)
No te los compres - Don't buy them for yourself.
No se los compres - Don't buy them for him/her/you (Ud.)/them/y'all (Uds.)
In the 2nd sentence "se" has replaced one of these: lo, la, los, las, which as the questioner, rabbitwho, explained about replacing the direct object pronoun with "se" when you have a direct object pronoun and an indirect object pronoun together.
