There are several complex conjunctions in Croatian that have two forms — one with što, and another with da. For example:
kao što (+ clause) as
kao da (+ clause) as if
The main difference is that forms with što refer to something that has happened, or will happen for sure (at least, what is expected to happen), and ones with da to something that did not happen, or is not expected to happen.
Vruće je kao što je bilo prošli tjedan. It's hot as it was the last week.
Vruće je kao da smo u Africi. It's hot as if we were in Africa.
The first sentence compares the heat to something that really happened, and the second one to something obviously not true. You can say the second sentence while in Africa only if you're joking.
Another situation where we have što vs. da is with comparison conjunction nego, when used with a clause:
nego što (+ clause) than
nego da (+ clause) than (something imagined)
The combination nego da is only used to compare to something unreal, imagined, while nego što compares to another, existing action or state:
Hotel je bolji nego što sam očekivao. The hotel is better than I've expected.
Another complex conjunction which shows such duality is umjesto:
(desired event) umjesto što (real event)
(real event) umjesto da (imagined event)
English here uses only instead.
When you look more carefully, the reason and purpose clauses follow a similar pattern:
zato što (+ clause) because
(zato) da (+ clause) so that
The correspondence is not perfect, for two reasons: first, zato is used in purpose clauses only for emphasis: only da is normally used. Second, purpose clauses are restricted to the present tense only.
(the rest is coming soon...)