ぜんぜん / 全然【ぜんぜん / ぜんぜん】

ぜんぜん (全然) means "not at all". It goes before a verb or adjective in the negative, and the sentence still has to end in ない or ません: ぜんぜん only makes the negative stronger.

• × ぜんぜんわかります → ぜんぜんわかりません

An い-adjective takes its own negative, 〜くない; a な-adjective or a noun takes じゃない or the more formal ではない.

• × ぜんぜん熱いじゃない → ぜんぜん熱くない

ぜんぜん or あまり

あまり with a negative is "not very", "not much": some, but less than expected. ぜんぜん is "none at all". まったく with a negative means the same as ぜんぜん and sounds more formal; it is the one you meet in writing.

On its own, ぜんぜん answers a question with "not at all", and the negative is understood.

With an affirmative in casual speech

In casual speech ぜんぜん also goes before an affirmative, where it brushes a worry aside: the other person expects a problem, and you say there isn't one. ぜんぜんだいじょうぶ大丈夫 and ぜんぜんいい are the everyday cases.

In writing and formal speech, keep ぜんぜん with a negative. ちがう違う counts as a negative here, so ぜんぜんちがう違う, "completely different", is fine anywhere.