What it means
な placed after a verb in its dictionary form gives a strong prohibition: “Do not ...” or “Don’t ...”. It is direct and forceful, so it is common in signs, warnings, orders, and speech from someone with authority. It can sound harsh in ordinary conversation. The polite equivalent is ～ないでください, which expresses the same prohibition more respectfully.
Usage note
Do not confuse sentence-final な with the sentence-ending な used to show emotion or seek agreement. The prohibitive な follows a dictionary-form verb and means “Do not ...”; it is not attached to a noun or adjective.
How to use it
ここで写真を撮らないでください。
Do not take pictures here.
ここで写真を撮るな。
Do not take pictures here.
Example sentences
緊急時以外は、このボタンを押すな。
Do not press this button except in an emergency.
試験中に辞書を使うな。
Do not use a dictionary during the exam.
作業中は、機械に近づくな。
Do not approach the machine while it is operating.
図書館の本を窓の近くに置くな。
Do not put library books near the window.