Some conjunctions (those little words that connect two part sentences) always (!!!) use the subjuntivo. These conjunctions have in their "nature" to express something unreal or uncertain. Again the thumb rule works.

Se lo he dicho para que aprenda lo que hay que hacer. = I told him that he learns what (he has) to do. explanation: para que expresses an intention, something that has not yet become real

En caso que venga, le doy el dinero. = In case he comes I give him the money. explanation: In case expresses clearly that it is still uncertain that the guy is coming,

Hace como si ganara mucho dinero, pero la verdad es que es tan pobre como nosotros. = He pretends (direct translation: does as if) he earned a lot of money, but in reality, he is as poor as we are. explanation: as if shows that it is not real