"It doesn't fly, but hop(s)"

Quick question: "It does not fly, but hop" or "It does not fly, but hops". I'm certain, it is the first one, my teacher claims, the second one is correct. Which one is correct and why?

It is also worth adding to what FumbleFingers has just said that the reason your teacher does not like the first option (if indeed it is an option!) is probably due to how it sounds with regards conventions of pluralisation. Just think about this sentence, "They do not fly, but hop". It sounds correct because the 'hop' tells the reader that the subject of the sentence is a group of things and not just one thing. It is the same principle as the difference between "She is an engineer and loves her job" and "She is an engineer and love her job". You probably wouldn't want to argue that 'love her job' is shorthand for 'she does love her job'; it does not sound correct.

There's also something to be said here for emphasis. By adding the proposed shorthand 'but it does hop', it seems like the author is making the point that the object (an animal I presume) is not all that disadvantaged in its mobility. By removing the 'does' the emphasis is no longer present. This is not taken care of with the replacement of 'hops' but the point I'm making is that if you are wanting to emphasise then it makes sense to use 'does' and not 'hop' unaccompanied by its emphasising partner.

Whilst I agree with FumbleFingers that it is overly close-minded to restrict language in this way, I assume that you are wanting to understand conventions and it is not, as far as I know, conventional to say that 'but hop' is shorthand for 'but it does hop'. I believe your teacher is referring to this distinction. Generally, 'shorthands' have to be learnt through experience and cannot be assumed logically.