Indicates that something is indispensable or cannot be done without; used to say that something is absolutely necessary.
水分補給がなくてはいけません、特に暑い日は。
Hydration is indispensable, especially on hot days.
この薬は、症状を抑えるために常に持っていなくてはいけません。
You must always carry this medicine to control the symptoms.
許可証がなくてはいけませんから、忘れないでください。
You can't do without the permit, so don't forget it.
Follows the negative base of a verb to express obligation or necessity—equivalent to 'have to' or 'must' (polite/neutral).
明日は早く行かなくてはいけません。
I have to go early tomorrow.
宿題を今日中に終わらせなくてはいけません。
You have to finish your homework by today.
健康のために野菜を食べなくてはいけません。
You have to eat vegetables for your health.