な

な after the dictionary form of a verb is a blunt "don't": さわる触るな, "don't touch". It is the plain negative command, and it sounds rough – a shout, an order, a warning sign.

Dictionary form, not ます-stem

な on the ます-stem is a different word: a casual "do it", short for なさい. One sound apart, the two mean opposite things. 来るな is "don't come"; 来きな is "come here".

• × 食べな (for "don't eat") → 食べるな

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な is for people close to you or below you, for anger, and for emergencies. To a stranger or anyone above you it is rude. The polite request is ないでください, and the casual one among friends is ないで.

Adding よ – ゆう言うなよ – takes some of the edge off and makes it closer to a plea between friends.