N5
ないほうがいい
It'd be better not to, Shouldn't do
あそこは危ないから、行かない方がいいよ。
That place is dangerous, so you'd better not go there.
Explanation
「〜ないほうがいい」 means 'you had better not …': 「無理をしないほうがいいです」 (You'd better not push yourself), 「そんなものは食べないほうがいい」 (You'd better not eat things like that). It is the reverse of 「〜たほうがいい」: not doing it is better than doing it, avoiding discomfort, loss or risk. The tone is gentle and sincere.
Mixed up with
Positive advice takes the た-form and negative advice the ない-form: 「行ったほうがいい」, 「行かないほうがいい」. Do not copy the positive pattern into 「なかったほうがいい」; that is a regret, 'I wish I hadn't gone', and means something quite different.
Compared with 「〜てはいけない」: 「〜ないほうがいい」 is well-meant advice (better not go); 「〜てはいけない」 is a forced prohibition (you must not go).