Expresses a strong necessity or obligation, equivalent to 'must do' or 'have to do'. It implies that not performing the action or not being in the state is unacceptable.
Verb [なくて-form] + は + ならないPrimary
い-Adjective [なくて-form] + は + ならない
な-Adjective + で + なくて + は + ならない
Noun + で + なくて + は + ならない
This phrase means you absolutely must do something. It shows that not doing it is not okay. You have no other choice.
健康を維持するためには、バランスの取れた食事をしなくてはならない。
In order to maintain good health, you must eat a balanced diet.
留学の準備で、たくさんの書類を集めなくてはならなかった。
I had to collect a lot of documents in preparation for studying abroad.
この契約書は、法的に有効でなくてはならない。
This contract must be legally valid.
全ての要求に応えなくてはならないわけではない。
It is not the case that you have to comply with every single demand.
チームの一員である以上、他のメンバーに協力的でなくてはならないと私は思う。
As long as you are a member of the team, I think you must be cooperative with the other members.
This phrase means something *must* happen. If it does not happen, that is not okay. It is like a rule that you cannot break.
This is like 'must do'. But it sounds more formal. It is also stronger than saying 'should do'. It means not doing it is not okay.
Watch out: Many people mix up "must do" and "must not do." "〜なくてはならない" means you must do something. "〜てはならない" means you must not do it. The small word "なく" changes everything. It tells you the verb is negative. For example, "食べなくてはならない" means "must eat." "食べてはならない" means "must not eat." Look for "なく" to know the difference.
More formal alternatives(3)
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