～なくてはならない "Must do..." vs ～なくてはいけない

Introduction

Way back in Elementary (N5), you learned the double negative「〜なくてはいけない」to express "something must be done". Today, we're going to cover this grammar point's distant, more formal cousin: 「なくてはならない」.

You must not give up!

Structure

• 

  negative, casual form

  the negative form of the casual/plain form, can also be called ない-form (e.g. {食|た}べる → {食|た}べない, {飲|の}む → {飲|の}まない).

  verb(ない ➔ なくてはならない)
  Example:
  → 急いそがなくてはならない。
  "You must hurry.”

In contrast to なくてはいけない, なくてはならない is slightly more formal, which means you'll see it more often in written Japanese, and other situations that require more formal speech. We'll contrast it with its casual alternatives later, but for now, let's get to know our new friend!