Learn Swedish: Eftersom ödet är hemligt måste vi tolka vad hon säger. - Since destiny is secret, we must interpret what she says.

Here, vad acts as an independent relative pronoun meaning "that which" or "the thing that". It connects the main verb tolka (interpret) to hon säger (she says). You can think of it working exactly like the English word "what" when it joins clauses together rather than asking a question.

Why is the word order måste vi instead of vi måste?

This is a classic example of the V2 (verb-second) rule! The entire first part of the sentence, Eftersom ödet är hemligt, is a subordinate clause that counts as the first position. To keep the main clause's verb in the second position, the verb måste has to come right after the comma, swapping places with the subject vi.

Why does the adjective hemligt end in a -t?

Hemligt (secret) is describing ödet (the destiny). The base word is ett öde. Because it is an ett word, the adjective describing it must also take the neuter form by adding a -t to the base adjective hemlig.

Could I start this sentence with för att instead of eftersom?