Grammar Info
N4 Lesson 8: 12/18

must, have to

ないとならない is generally not used and is best avoided

Verb[ない]+ + (いけない(1)

(1) だめ
Verb[ない]+ + (いけません(1)

(1) だめです
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使用域 タメ語
As an extension of the ‘if’ meaning of , attaching the auxiliary verb ない to the verb before it will express ‘if (A) is not done, (B)’. (B) is most frequently いけない ‘cannot go’, which makes ないといけない a double negative phrase, which translates to ‘must do (A)’, or ‘have to do (A)’.
いけない may be replaced with だめ, to make the phrase more casual, but ならない is generally not used with ないと.
  • ライト君(くん)悪(わる)んだから、リュウ君(くん)に謝(あやま)らないといけないでしょう
    Raito-kun, you are in the wrong, so you have to apologize to Ryu-kun, right?
  • 今日(きょう)勉強(べんきょう)ないとだめから、明日(あした)
    I have to study today, so how about tomorrow?
ないといけない is regularly abbreviated to ないと, but has exactly the same meaning in these cases.
  • 明日(あした)テストある?!勉強(べんきょう)しないと
    There is a test tomorrow!? I have to study.
  • もう7時(じ)じゃん?準備(じゅんび)しないと
    Oh, it's already 7? I need to get ready.
As has the ‘and’ nuance, it gives ないと a level of strength that is not seen in similar phrases like なければいけない, or なくてはいけない. Because of this, ないと is often used to highlight things which absolutely ‘must’ be done (obeying the law, adhering to customs, using common sense, etc.).
  • 歩行者(ほこうしゃ)信号機(しんごうき)守(まも)らないといけない
    Even pedestrians have to obey traffic lights.
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"You have to keep your promise!"
"Why do I have to pay taxes?"
"There are lots of children, so I have to do laundry thirty times a week."
Tae Kim's Japanese Grammar Guide
Page 142