"He is a man who knows what he wants."
Translation:Er ist ein Mann, der weiß, was er will.
Let's break down the sentences to see which parts of sentence form structures:
ENG: 1: He is a man / 2: who knows / 3: what he wants.
DEU: 1: Er ist ein Mann, / 2: der weiß, / 3: was er will.
1: main clause
2: relative clause
3: content clause referring to 2
Here, German and English syntax run completely parallel.
Verbs in the German sentence:
In the main clause (1) the verb is in second position (as per v2-rule), in both the subordinate clauses (2 and 3) the respective verb is in the respective last position.