Imprudently is how you do something when you are not using good sense. It sits on the action, not on the person: you spoke imprudently, you spent imprudently, you drove imprudently. The meaning matches imprudent, only it describes the doing. It is formal. In everyday talk you are more likely to hear “that wasn’t smart” or “you should have been more careful.” Use it when the trouble came from not looking ahead — not from bad luck, and not from wanting to hurt anyone. Though actually, after a bad result, people sometimes say imprudently about a choice they simply disagree with.