"This box will serve as a chair."
Tłumaczenie:To pudełko posłuży jako krzesło.
Bedzie służyć albo bedzie służyło... Obie opcje są poprawne Duolingo :-/
'This box will be used as a chair' – generally, the meaning of the sentence is almost the same, but Duolingo is based on teaching through induction(ie. learning by example) and that requires from us to more closely follow the actual semantic equivalence of words, because the example sentences are supposed to teach you the meaning of the words that are used in it.
If we would allow drifting so far from the actual meaning, there is a risk that some users would think that 'to use' has the same meaning as 'to serve'(ie. that „użyć” means the same as „służyć”) and then will justly ask us why we don't allow „Będzie służył w armii” = 'He will be used in the army' and we really wouldn't like that at all. ;)