N5
なくてはならない
Must do, Have to do
毎日勉強をしなくてはならない。
You must do your studies every day.
Explanation
「〜なくてはならない」 also means 'must …', literally 'it won't come about if you don't'. It leans toward duties that cannot be avoided: objective laws, the law, social norms: 「国民は税金を払わなくてはならない」 (Citizens must pay tax). The tone is solemn, used in written articles, news and formal speeches. The polite form is 「〜なくてはなりません」.
Mixed up with
「〜なくてはいけない」 stresses personal feeling, everyday human obligation and common sense, mostly in speech; 「〜なくてはならない」 stresses social responsibility, law and objective rules, mostly in writing.
Both are 'must' in English, but in exam reading passages, official documents and papers, almost always 「〜なくてはならない」 (or 「〜なければならない」) is used.
「いけない」 comes from 「行く」 and leaves room for human leniency; 「ならない」 comes from 「成る」 and is about whether something can hold objectively.