Korean Job Discussion Forums

[jugbandjames]

In "You certainly work hard, don't you? (You must work hard, don't you?)" certainly is an adverb, so without an auxiliary verb, you have to use do in the tag question. Even though must and certainly have similar meanings, they're different parts of speech so the substitution isn't valid. Since must is an auxiliary verb for both meanings, I think it should be repeated in the tag question for both. There may be some other reason why you use don't, but it's not because you can substitute "certainly" for "must", because you can't. They share meanings but not syntactic categories.