ないといけない

ないといけない means "must", "have to". It goes after the ない-form of a verb: literally "if I don't do it, it won't do". いけません at the end makes it polite.

The verb changes to its ない-form first: いく行く → いかない行かない → いかない行かないといけない.

• × 行くないといけない → 行かないといけない

ないといけない or てはいけない

Both end in いけない, but they are opposites. ないといけない says you must do something; てはいけない after the て-form says you must not.

ないといけない or なければならない

Both mean "must". ないといけない is the one you hear in conversation, often about what a situation demands right now. なければならない sounds more formal, and is the usual choice for rules and in writing.

Usage notes

These go beyond the full form.

Shortened to ないと

In casual speech the ending often drops, leaving just ないと.

Asking whether you have to

A question with か asks whether something is required.