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prohibitive form : dictionary form + na

To command someone not to do something in Japanese is very easy. Simply add な after the plain form of a verb.

For example, いく (iku) becomes いくな (ikuna - don't go), いう (iu) becomes いうな (iu na - don't talk), たべる (taberu) becomes たべるな (taberu na - don't eat), する (suru) becomes するな (suru na - don't do), くる (kuru) becomes くるな (kuru na - don't come).

So to answer your question on the negative imperative-form for "Don't do that!", it would be するな (suruna). The imperative-form for "Do that!" would be しろ (shiro).

One common situation where this form of speech is used is when the speaker has authority or a supervisor role over the listener. 

泣くな！
naku na!
Don't cry! 

はしるな！
hashiru na!
don't run!

寝るな！
Neru na
Don’t sleep

これを食べるな！
kore o taberu na!
Don’t eat this.

ここに座るな！
koko ni suwaru na!
Don’t sit here.

きかいをさわるな！
kikai ni sawaru na!
Don’t touch the machine!

心配するな！
Shinpai suru na 
Don't worry about it.

運転するなら飲むな！
Unten suru nara nomu na
If you drive don't drink

If you go to Japan, you'll notice that imperative-form is also being used in some of the Japanese signs. For example, you may see the sign 入るな (hairu na), which is a "Do Not Enter!" sign, prohibiting you from entering an restricted area.
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