What's the correct construction?
A bear! Better not (to) move or it would devour her.
Should the to be there? Why or why not?
The "better not move" sentence seems like an imperative (you are commanding the other party---after all, there's a bear). Placing the "to" after the not sounds a bit awkward to my ear but it is less imperative, like a suggestion. Perhaps "It would be better not to move..." would be a way to rephrase the sentence if the bear were far away.