ないほうがいい details

write + not + had better not do

kakanai hou ga ii

had better not do; should not (advice)

meaning: had better not write

そのこと、まだ言わいわないほうがいいよ。

sono koto, mada iwanai hou ga ii yo.

You had better not say that yet. casual

熱ねつがあるなら、今日きょうは出かけでかけないほうがいい。

netsu ga aru nara, kyou wa dekakenai hou ga ii.

If you have a fever, you had better not go out today. neutral

Examples

had better not; it's wiser not to — advising against an action

そのこと、まだ言いわないほうがいいよ。

sono koto, mada iwanaihougaii yo.

You had better not say that yet. casual

熱ねつがあるなら、今日きょうは出でかけないほうがいい。

netsu ga aru nara, kyou wa dekakenaihougaii.

If you have a fever, you had better not go out today. neutral

会議かいぎでは、その話題わだいには触ふれないほうがいいと思おもいます。

kaigi de wa, sono wadai ni wa furenai hou ga ii to omoimasu.

I think it would be wise not to touch on that topic in the meeting. formal

Always plain ない before ほうがいい — ×なかったほうがいい isn't advice. Past goes on the tail instead: 言わないほうがよかった, "it would have been better not to say it."

てはいけない
ないほうがいい advises against and leaves the choice with the listener; てはいけない removes the choice by rule — a doctor softens 飲んではいけません into 飲まないほうがいいです only when it truly is just advice.

たほうがいいplain-antonym
Polarity flips inside the frame with an asymmetry worth drilling — advice to do rides た (行ったほうがいい), advice to refrain rides plain ない (行かないほうがいい), never ×なかったほうがいい.