Learn Hardcore Dutch: Omdat ik morgen vroeg moet opstaan, doe ik het licht nu uit. - Because I have to get up early tomorrow, I am turning the light off now.

Because omdat is a subordinating conjunction; it pushes the finite verb (and any modals/auxiliaries) to the very end of its clause. You end up with a verb cluster where the modal moet directly precedes the infinitive opstaan.

Why is uit at the end of the main clause, as in doe ik het licht nu uit?

The verb uitdoen is separable. In a main clause you split off the prefix uit, put the finite verb (doe) in the second position, and place the separated prefix at the very end.

Why does the main clause read doe ik het licht nu uit instead of ik doe het licht nu uit?

Dutch main clauses follow the V2 rule: the finite verb must occupy the second slot. Because the subordinate clause (everything before the comma) counts as the first slot, you invert subject and verb in the main clause, yielding doe ik rather than ik doe.

Why is the time expression morgen vroeg and not vroeg morgen?

Time expressions in Dutch typically list the time unit (here morgen = “tomorrow”) first, followed by the modifier (vroeg = “early”). Hence morgen vroeg (“early tomorrow”) is the natural order.

Why does the subordinate clause read omdat ik morgen vroeg moet opstaan instead of omdat morgen vroeg ik moet opstaan?

In clauses introduced by omdat, the only rigid rule is “verb at the end.” However, for a neutral and clear word order, Dutch speakers place:

the conjunction (omdat)
the subject (ik)
the time adverbial (morgen vroeg)
the verb cluster (moet opstaan)
Putting morgen vroeg before ik is grammatically possible but sounds marked or overly emphatic.

Could I use want instead of omdat in this sentence?

Yes—but watch word order:

omdat (subordinating): forces verb-final order in its clause.
want (coordinating): does not affect word order (each clause remains V2).
So with want you’d write:
Ik moet morgen vroeg opstaan, want ik doe het licht nu uit.
Notice moet stays in V2 in the first clause.

Why does opstaan stay together in the subordinate clause while uitdoen splits in the main clause?

In subordinate clauses all verbs and separable prefixes form one block at the end. Thus you see moet opstaan (modal + intact infinitive). In main clauses, separable verbs split: the finite verb (doe) goes in second position and the prefix (uit) moves to the end.

Could I say zet ik het licht nu uit instead of doe ik het licht nu uit?

Yes. uitzetten is another separable verb meaning “to switch off.” Both het licht uitdoen and het licht uitzetten are common. uitzetten often emphasizes operating a switch, while uitdoen is a bit more general (“to turn off”).