The negative command form of a verb in Japanese (imperative?), for example, ‘Don’t talk!’ is formed using the dictionary form of a verb, V.dict and adding ‘な’. In the previous example, this would be:

話すな (はなすな)

Other examples:

行くな (いく) Don’t go!
来るな (くる) Don’t come (nearer etc…)!
食べるな (たべる) Don’t eat!
走るな (はしる) Don’t run!

This is a very strong way of speaking, i.e. fighting talk, giving commands and so on… in other words, to be used with caution. However, as with many things, this rule is not set in stone. Depending on the situation and tone of the speaker, this form can be used in different ways. For example:

…泣くなよ… Don’t cry…

In this example the speaker is being reassuring and comforting the person who is crying. Today I heard this type of usage and tone for the first time.