〜なくてはいけない
must, have to
Verb ない-form − い + くてはいけない / いけません
Sign up free to review this point on a spaced schedule.
なくてはいけない is another "must", built from the ない form: 来ない → 来なくて ("if (you) don't come") + は + いけない ("it won't do"). 薬を飲まなくてはいけません, "I have to take my medicine".
It means the same as なければならない. It sounds a little more personal and spoken, where なければならない leans towards rules and general obligations.
In speech it shrinks to なくちゃ: 行かなくちゃ, "I've got to go". Along with なきゃ and ないと, these short forms are what you'll hear most in conversation and anime.
Be careful to build it on ない → なくて, not ないで: 行かないではいけない is wrong.
Register: Spoken shortcut: なくちゃ (行かなくちゃ).
Examples
明日は早く起きなくてはいけません。
I have to get up early tomorrow.
もう帰らなくてはいけない。
I have to go home now.
今日は宿題をしなくてはいけません。
I have to do my homework today.
明日、病院に行かなくてはいけません。
I have to go to the hospital tomorrow.
毎日、犬と散歩しなくてはいけない。
I have to walk the dog every day.