Breakdown of Wat de leugenaar beloofde, was een list.
zijn
to be
een
a/an
beloven
to promise
wat
that / which (relative)
Questions & Answers about Wat de leugenaar beloofde, was een list.
How does wat work in this sentence since there is no noun before it?
It acts as a free relative pronoun. Instead of referring back to a previously mentioned noun, wat stands on its own to mean "the thing that" or "that which". The entire clause Wat de leugenaar beloofde functions as a single block that acts as the subject of the main verb was.
Why does the second half of the sentence begin immediately with the verb was?
This is a strict application of the Dutch verb-second (V2) rule. The entire relative clause Wat de leugenaar beloofde occupies the first position in the main sentence. To keep the main verb in the second position, was must come immediately after the comma.
Could I use truc instead of list for "trick" here?
You could, but list is much more appropriate in this context. A truc is usually a magic trick, a physical stunt, or a clever life hack. A list specifically refers to a ruse, scheme, or deceptive stratagem meant to mislead someone, which perfectly fits the actions of a leugenaar (liar).
Why is the past tense beloofde and not beloofte, since the stem beloof ends in an 'f'?