〜ないほうがいい

1. 1Advises the listener, or people in general, that it is better not to do a certain action.Ex. 1
2. 2With an adjective or noun in the negative, expresses that it would be better if something were not in a certain state.

A common, neutral way to give advice. Add です for politeness, or ですよ to sound friendly, as in example 1. It often comes with a reason, marked by から or ために. It is milder than a prohibition and stronger than a simple suggestion.

• 〜たほうがいいN4「たほうがいい」 follows the た-form and urges the listener to do something, while 「ないほうがいい」 follows the ない-form and urges them not to; they share the same tone and register but are opposite in polarity, so they aren't interchangeable. Negative advice cannot be phrased as 「なかったほうがいい」.
• てはいけないN5「てはいけない」 is an explicit prohibition used in rules, notices, and warnings, with a firm tone, whereas 「ないほうがいい」 is only advice that leaves some leeway and cannot express rule-based prohibition. 「てはいけない」 follows the て-form and can also attach to adjectives and nouns.
• ないでくださいN5「ないでください」 directly asks the listener not to do something and can also express prohibition on notices and in regulations. 「〜ないほうがいい」 is only advice given in the listener's interest, leaving them room to decide for themselves, and cannot express rule-based prohibition.
• べきではないN3「べきではない」 asserts on grounds of reason or morality that something shouldn't be done; it is heavier in tone, can be used to criticize others, and in the past form expresses regret. 「〜ないほうがいい」 is just advice offered with the listener's interests in mind, while 「べきではない」 attaches to the dictionary form.
• なくてもいいN4「なくてもいい」 means it's fine not to do, have, or be something and carries no discouragement, whereas 「ないほうがいい」 urges the listener not to do it, so the two aren't interchangeable. 「なくてもいい」 is formed by changing the final 「い」 of the ない-form to 「くてもいい」, and it also attaches to adjectives and nouns.

〜たほうがいい recommends doing something, while 〜ないほうがいい recommends not doing it; てはいけない is a prohibition, ないでください is a request not to do something, and べきではない states a stronger duty-like 'should not'. なくてもいい means 'don't have to', expressing no necessity rather than advice against acting.