Learn Swedish: Han förstår inte vad stjärntecknet avslöjar. - He does not understand what the zodiac sign reveals.

Som is a relative pronoun that points back to a specific noun you just mentioned. Vad is an independent relative pronoun, meaning it stands on its own and translates to 'what' or 'that which'. Since there is no specific noun before it that it refers back to, we use vad.

Is vad the subject or the object in the second part of this sentence?

In this sentence, vad is the object. The subject of that clause is stjärntecknet (the zodiac sign), and the verb is avslöjar (reveals). If vad were the subject (for example, 'He does not understand what is happening'), you would have to add som and say vad som. Since it is the object here, just vad is correct.

If I wanted to say 'what the zodiac sign does NOT reveal', where would inte go?

It would go before the verb, making it vad stjärntecknet inte avslöjar. The second half of this sentence is a subordinate clause. In Swedish subordinate clauses, sentence adverbs like inte must come before the verb (this is often taught as the BIFF rule).

How is the word stjärntecknet built?

It is a compound of en stjärna (a star) and ett tecken (a sign). When they are joined, the first 'a' is dropped. The gender of a compound word always comes from its final part, so it acts as an 'ett' word. Adding the definite suffix gives us stjärntecknet (the zodiac sign).