Grammar Point
expressions (phrases, clauses, etc.)
exp 
adjective (keiyoushi)
adj-i 
word usually written using kana alone
uk 
はいけない

must not (do, be), should not, ought not to

after the -te form of a verb or adjective

Verb て form + はいけない

This grammar point is used to express a strong prohibition or a command not to do something. It indicates that an action is forbidden or that a state should not exist.

Verb

When used with verbs, it creates a direct command or rule stating that an action is prohibited.

You must not enter here.

You should not eat that.

You must not forget your homework.

Note

In casual speech, ては often contracts to ちゃ, making the phrase ～てはいけない become ～ちゃいけない. While this grammar point is used for general prohibitions, it is often used by people in authority or when speaking to someone younger.

Adjective て form + はいけない

This formation expresses that a certain state or quality is undesirable or should not be the case.

い-Adjective

For い-adjectives, change the final い to くて before adding はいけない.

It is not good to be noisy.

な-Adjective

For な-adjectives, add で before はいけない.

It is not good to be mean.

Note

When using adjectives, this grammar point is less common than with verbs and often implies that the person should not be acting in a certain way or that a condition is unacceptable.

Words: 
行けないいけない
Radicals of radicals