Translation:He returned yesterday but I still have not seen him.
As does... He returned yesterday but still I have not seen him... quite frustrating
You are absolutely correct. Whoever determines the correct answer has a very limited grasp of the nuances of English!
He returned yesterday but i have not seen him still has exactly the same meaning!!
ni fhaca is surely "didn't see" as "have not seen" would be a different construction
"didn't see" should be accepted, but it is the same construction in Irish. This is because the English uses the past perfect in certain situations, whereas Irish only uses the past. So, sometimes (like here), it's the best translation.
It helps if you quit trying to think of one-to-one correspondences between the languages.
Does "fhaca" mean "looked" or "saw"? I thought the past tense of the verb "to see" was chonaic