いけない
ここでタバコを吸っちゃいけない。
You must not smoke here.
must not do; should not do; forbidden
lateness is not allowed.
遅刻してはいけない。
You must not enter this area.
この場所に入ってはいけない。
Used in prohibitive contexts, often with てはいけない (te wa ikenai) or just いけない. More casual than いけません.
no good; bad; wrong
This food tastes awful.
この食べ物は全然いけない。
Evaluative sense, often used to describe food or situations as unpleasant or unsatisfactory.
in trouble; in a bad situation
I broke a window and now I'm in trouble.
窓を割ってしまって、いけないことになった。
Often used in the phrase いけないことになる (ikenai koto ni naru) to indicate a negative outcome or getting into trouble.
Related Phrases
Pro Tip
Casual vs. Polite
Use いけない in casual settings with friends or family. Use いけません in formal or polite situations, such as with strangers or in professional contexts.
Gold Rule
Prohibition Structure
To express 'must not' in Japanese, use the て-form of a verb + は + いけない. Example: 食べてはいけない 'must not eat'.
Word Origin
From the verb いく (iku, 'to go') + negative auxiliary ない (nai). Literally 'not to go', but evolved into a general prohibition or negative evaluation.
Usage Notes
いけない is the informal form of いけません. It is commonly used in speech and casual writing. In formal contexts, いけません is preferred. The phrase てはいけない is a standard way to express prohibition (e.g., ここで食べてはいけない 'You must not eat here').