Translation:There is a fish inside the mouth of that cat.
More natural to say in English: That cat has a fish in its mouth. or There is a fish in that cat's mouth.
These are better translations than Duolingo's. Literally: that cat's mouth's inside has a fish in it.
Where does the "that" come from? The drop-down hint only said, "the." I said "There is a fish in the cat's mouth."
那个，那只， 那本，那条， 那张 or any other quantifier is that (or those if plural) WHEREAS 这个，这只， 这本， 这条， 这张 or any other quantifier is this (or these)
Not really. The drop down hint does say that 只 can be a classifier, which is also what my dictionary calls it (i.e. a classifier for birds and animals). However you would need to use the context to identify that was the case here.
What is the difference between in the mouth and inside the mouth? Why did it mark me wrong for writing "in" instead of "inside"?