Do you have to use "ficou" in this case? Does it not make sense to say "ele foi doente?"
No. In this case "ficou" is the verb to be used.
Is there no difference between he does not walk and he has not walked?
In sentences with "desde" the Portuguese present tense is used but the best translation uses the present perfect in English. See my comment in this discussion: https://www.duolingo.com/comment/1041145.
Is it possible to say:"....desde que ele estava doente"?
Yes, it is also right but less common!
I translated since as porque and was marked wrong. Could this frase have this meaning too? (Not since some moment but since = 'the reason why')
A possible answer. For this "since" we also use "já que", "uma vez que", =)
Thanks a lot, two more useful conjunctions in my vocabulary! :)
Is the "que" in this sentence totally necessary?
yes, sure. When "that" works as a relative pronoun (the same for who, which)or to join 2 sentences, in Portuguese it cant be omitted!
I don't see where "that" fits anywhere into this sentence? I just wrote "desde ele ficou doente", which apparently wasn't right.
Since + noun/pronoun = desde que + noun/pronoun,
I wrote "caminhava" and "estava doente" and it was marked as wrong...
"Tinha ficado doente" works better.