Structure
Verb［ない］+ と + （いけない(1)）
(1) だめ
Details
Casual
About ないと
As an extension of the 'if' meaning of と, 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.
Caution
As と 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.).
Related
Examples
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「約束を守らないといけません！」
'You have to keep your promise!' (must)
「なぜ、税金を払わないといけないのですか？」
'Why do I have to pay taxes?' (must)
「子沢山だから、一週間に三十回は洗濯しないとだめ。」
'There are lots of children, so I have to do laundry thirty times a week.' (must)
「合格したらまず両親や先生に伝えないと。」
'If I pass, I have to tell my parents and teacher first!' (must)
「ピーナッツにアレルギーがあると言っておかないといけないんだ。」
'I have to tell (you) in advance that I am allergic to peanuts.' (must)
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〜と & 〜ないと conditional
Maggie Sensei
Breakdown and Examples
Nihongo Master
Verb[ない] + と conditional
Tae Kim
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Tae Kim's Japanese Grammar Guide
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ないと – Grammar Discussion
Most Recent Replies (20 in total)
discopatrick
@Yametekure - This question intrigued me… now that I think about it, I’m not sure I’ve ever spoken or read the past tense of any of these “must do” phrases. Do they exist?
And what would the translation even be? Something like “at that time (in the past) I was compelled to do X” or “I had to do X” or “not doing X would not have gone”?
Great question!
discopatrick
Also, could you expand on this please BunPro?
casual
Sure, why not. Constructions like “it had to be done”, “I had to” come up If you are telling a story that happened in the past, or in something like “I know I had to do X, but something else happened”.