Grammar Info
N5 Lesson 9: 8/13

なくてはいけない
Must do, Have to do

Also used to express duty or necessity. If you are curious about なくて conjugation

Structure
Casual
Formal
Verb[ない+ なくては + いけない
Verb[ない+ なくちゃ + いけない
Verb[ない+ なくては + いけません
Verb[ない+ なくちゃ + いけません
Details
Part of Speech Expression
Word Type Auxiliary Verb
Register Formal
品詞 表現
単語の種類 助動詞
使用域 硬い
Information
Much like てはいけない, なくてはいけない states that something ‘must not’ be done. However, this is where the similarities end. なくてはいけない is an example of a double negative in Japanese, and actually has the meaning ‘(A) must be done’, or ‘must do (A)’.
While this grammar point is usually translated as ‘must do’, the literal translation is ‘must not, not do (A)’. This double ‘not’ is where many learners have difficulties. いけない simply means ‘cannot go’, and comes from the negative potential form of 行(い)く.
To use this structure, simply create the negative form of a verb, and then convert ない to なく (the conjunctive form of an い-Adjective), followed by . After this, is added, and then finally いけない.
  • 今日(きょう)9時(じ)寝(ね)なくてはいけない
    Today, I must sleep at 9. (I must not, not do it)
  • 明日(あした)彼女(かのじょ)誕生日(たんじょうび)プレゼント買(か)わなくてはいけない
    I must buy my girlfriend's birthday present tomorrow. (I must not, not do it)
  • 明日(あした)友達(ともだち)来(く)るから、家(いえ)掃除(そうじ)なくてはいけない
    Since my friend is coming tomorrow, I must clean my house. (I must not, not do it)
いけない is considered to be the most common structure used with the ‘must’, and ‘must not’ grammar points, and is natural in both speech, and written Japanese. Alternatively, ならない may be used in formal speech, but is much better suited to writing. だめ is also acceptable, but much more casual than either いけない or ならない.
  • 朝食後(ちょうしょくご)薬(くすり)飲(の)まなくてはならない
    After breakfast, I must drink my medicine.
  • 野菜(やさい)食(た)べなくてはダメ
    You must also eat vegetables.
Caution
The ては in なくては is often converted to ちゃ, as this is easier to say. なくちゃ may then be followed by いけない, or いけない may be omitted, without changing the meaning.
  • 土曜日(どようび)会社(かいしゃ)行(い)かなくちゃいけない
    I gotta go to the company on Saturdays too.
  • 今日(きょう)スーパー行(い)かなくちゃ
    Today, I gotta go to the supermarket.
These casual structures are very similar to words like ‘gotta’ in English.
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テスト前(まえ)に、よく寝(ね)なくてはいけない
Before a test, you must sleep well.
Play  
12月(がつ)なってから、暖房(だんぼう)つけなくてはいけない
Once December comes, you have to turn on the heating.
Play  
来週(らいしゅう)試合(しあい)があるのでたくさん練習(れんしゅう)なくてはいけない
Because I have a game next week, I must train a lot.
Play  
Books
Tobira
Page 43
Tae Kim's Japanese Grammar Guide
Page 142