Just curious about the use of the subjunctive in this statment.

Question

Hello, this is a more advanced topic but I've come back to review.Why do we use the subjunctive in this statement:Ustedes pueden comprar lo que quieran.Thanks.

Accepted Answer

Hola Marcos
This is to do with referring to something known or unknown by the speaker. Relative clauses can use the indicative or the subjunctive and this would be a relative clause introduced by lo que where the speaker is not sure/doesn't know what exactly is that they want to buy, therefore it's using the subjunctive.
If this same sentence was using the indicative: "...lo que quieren", this would indicate that the speaker knows what they want to buy, probably due to a previous conversation. 
We have a lesson on relative clauses where we explain this with subordinate clauses using "que" (not lo que) but it is the same idea. You can have a look here.
Saludos
Inma

Answer

Lo que - whatever - You don't know what the person is going to buy or if they are going to buy anything hence the use of the subjunctive.