Linking words that present something as hypothetical or as an aim take the subjunctive: encara que, malgrat que, per tal que, abans que. Note that perquè means "because" with the indicative but "so that" with the subjunctive, so the mood alone changes the argument.
Examples
- Encara que sigui car, val la pena defensar-ho.
- Even if it's expensive, it's worth defending.
- Ho explico així perquè tothom ho entengui.
- I explain it this way so that everyone understands it.
- Malgrat que hi hagi dades contràries, la tesi es manté.
- Even though there is conflicting data, the thesis holds.
- Cal decidir-ho abans que sigui massa tard.
- We must decide before it's too late.
The mistake to avoid
Perquè + indicative gives a cause (ho explico així perquè és complex = because it's complex); use the subjunctive only when you mean purpose.
Where this fits in
L'argumentació lògica, Catalan C1: Build a logical argument: claims, evidence, counterargument, persuasion.