"I drink coffee before I sleep."
Translation:Ik drink koffie, voordat ik slaap.
"voordat" is a subordinating conjunction. Subordinating conjunctions always force the verb in the subordinate clause to the end of the clause: "ik drink koffie, voordat ik het ontbijt eet." Additionally, if the independent clause comes after the subordinate clause, the independent clause's verb is forced to the beginning of the clause: "Voordat ik het ontbijt eet, drink ik koffie."
You can read about the "verb at the end" rule in the Tips and notes section of the Conjunctions skill, and you can read more about word order on this page, which is part of this Dutch grammar overview on Duolingo.
Gasan: I believe you're confusing inversion (which occurs when a sentence or main clause begins with something other than the verb) -that is, VSO-, with subordinate clause word order, SOV.
In this particular sentence, as in the subordinate clause there's no object (since slapen is an intransitive verb), it seems as if the word order remained the same as if it were an independent clause, but as soon as we change slapen for a transitive verb, we need to alter the word order, just as the previous user explained (Narion K or something).
Voordat is a contraction of 'voor' and 'dat' and could be translated as '(be)for(e) that'. Voor can be used here but it can also have several different meanings.
De appel is voor hem =The apple is for him
Ik sta voor het huis = I'm standing in front of the house
We/wij bidden voor(dat) we eten = We pray before we eat