"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.
OK I see that but if the English were 'He is a man who knows it' then it would be 'Er ist ein Mann der es weiß' (which is not the English word order)
Replace 'es' with 'was er will' and you have the order that was marked wrong.
I'm not sure I fully understand. Is '... der, was er will, weiß' completely wrong, or less common or what?