～なくてはならない

About

The 「～なくてはならない」 ("must do X") pattern is formed by combining a verb's negative て form (なくて) with the は particle and ならない ("will not become"). This literally translates to something like "As for not doing X, it won't do."

E.g. 勉強べんきょうしなくてはならない。 "I must study."

～なくてはならない is a formal way to say that one "must do" something and is often used in writing. In casual situations, other "have to" grammar should be used instead.

Summary

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  ～なくてはならない means "must do X"

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  ～なくてはならない is formal