Is "must certainly" redundant or incorrect? Or is it a valid way to stress the certainty of "must"?
Examples:
You must be wrong.
You must certainly be wrong.
"You must be wrong." - Implies no doubt or even the possibility of doubt. It's a politer or softer way of saying "You are wrong"
"You must certainly be wrong." - "certainly" emphasizes the belief of the speaker that the other person is wrong. However, depending on intonation, it could also be lessening the strength of "must" even further. ("Certainly, he couldn't be that fast? Could he?" vs "He certainly isn't that fast")
May sound redundant, but not incorrect. Probably used more spoken then written. You must really go. You must absolutely wait for me.