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
Raito-kun, you are in the wrong, so you have to apologize to Ryu-kun, right?
I have to study today, so how about tomorrow?
is regularly abbreviated to ないと
, but has exactly the same meaning in these cases.
There is a test tomorrow!? I have to study.
Oh, it's already 7? I need to get ready.
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.).