What if you don't like ending your sentences with a preposition?
You'll just have to ask Duolingo to accept the really dreadful sentence "For what is that spoon?". :-)
Not sure why 'that' and 'this' are not interchangeable here.
Because the speaker obviously isn't in physical possession of the spoon. If they were holding it, then "what is this spoon for" would be the correct version.
In Portuguese, is the structure always "for what is that (thing)" as opposed to "what is that (thing) for"?
Yes. You should place prepositions in the beginning of interrogative sentences.
Why is 'aquela' used, but not 'essa' or 'isso'?