This is a brief overview of prohibitive forms in the Japanese language. The prohibitive form is used to forbid someone to do something or to urge someone to refrain from doing something strongly.

Do not!

The rentaikei + な (na) construction is used to create strong prohibitives.

Nomu na.
Don't drink!

Noru na.
Don't drive!

Wataru na.
Don't cross (the road)!

Note: This form can be made more polite by using honorific verbs instead of the regular verbs. The ます (masu) form can also be used but is not as common.

Note: Be careful not to confuse "rentaikei + な" with "ren'youkei + な".

飲むNomu na.
Don't drink!

Nomi na.

Must not!

There are several ways to describe that you "cannot" or "must not" do something. This construction uses the conditional form followed by the negative of "to become" or "to be able to go".

行くとならない。 Iku to naranai.行くといけない。 Iku to ikenai.
行ってはならない。 Itte wa naranai.行ってはいけない。 Itte wa ikenai.
行けばならない。 Ikeba naranai.行けばいけない。 Ikeba ikenai.

All these variations translate as "You cannot go." or "You mustn't go." Of course, all variations can be made polite by adding ます (masu) to the ren'youkei conjugation of the final (main) verb.

Please, do not!

This construction is achieved by adding ないでください。 (nai de kudasai) to the mizenkei conjugation.

Sore wo shinai de kudasai.
Please don't do that.

Enpitsu de kakanai de kudasai.
Please don't write with a pencil.