Is hard-working hyphenated

Answer 1

Yes, it is a compound adjective and should be hyphenated. In that context, 'hard' is an adverb modifying 'working'. If you do not hyphenate it, you are using two separate adjectives, 'hard' and 'working'.

Answer 2

If you look at Merriam-Webster it cites 'Hardworking'. As A Brit, I would use 'hard-working' (as cited in the Oxford Dictionary). However, neither is incorrect.

Answer 3

Both can be used.