What it means
なくてはならない expresses necessity or obligation: someone must do something, or something must have a particular quality or condition. For verbs, use the negative nai-form with its final い removed, then add くてはならない. For example, 飲まない becomes 飲まな, producing 飲まなくてはならない. This pattern is stronger and more formal than simply saying that something is desirable. A polite version uses なくてはなりません.
Usage note
なくてはならない is often used for rules, duties, and requirements. Do not confuse it with なくてもいい, which means that something is not necessary.
How to use it
参加者は学生でなくてはなりません。
The participants must be students.
この教室は静かでなくてはなりません。
This classroom must be quiet.
この部屋は明るくなくてはなりません。
This room must be bright.
毎日薬を飲まなくてはなりません。
It is necessary to take medicine every day.
参加者は学生でなくてはならない。
The participants must be students.
この教室は静かでなくてはならない。
This classroom must be quiet.
この部屋は明るくなくてはならない。
This room must be bright.
毎日薬を飲まなくてはならない。
It is necessary to take medicine every day.