What it means
ないと expresses an "if not" or "unless" condition. It attaches to a verb's plain negative form and says that the result in the following clause will occur if the action is not done. It commonly describes a necessary action or an undesirable consequence, and the following clause may be familiar or polite independently of the conditional clause.
Usage note
ないと can also be used at the end of a sentence with the meaning "must" or "have to," especially in informal speech. In polite sentences, the conditional clause usually remains in the plain form while the following clause takes a polite ending.
How to use it
早く出ないと、電車に間に合いません。
If we don't leave soon, we won't make the train in time.
早く出ないと、電車に間に合わない。
If we don't leave soon, we won't make the train in time.
Example sentences
明日の朝までに申込書を提出しないと、参加できません。
Unless you submit the application form by tomorrow morning, you cannot participate.
暗くなる前に帰らないと、母が心配します。
If I do not return home before it gets dark, my mother will worry.
この薬を食後に飲まないと、十分な効果