I see now that the principal translation is 'properly', but it gives this as an alternative correct answer: "Who knows how to swim correct?" Surely, this ought to be "Who knows how to swim correctly?" given the context, since 'correct' is an adj. not an adverb. Shouldn't "Who knows how to swim correctly?" be the alternate form?
Yeah that certainly shouldn't be correct, it might have just been a slip up since we don't really differentiated between adverbs and adjectives in Welsh (as in we don't add anything to make it an adverb).