How do you ask someone to not do something?
For example, "Please don't buy that shirt. It's scary."
You would take the verb, transform it into short form (ending with -anai), and add でください。
to buy: かう
to buy in short form: かわない (Note that for U verbs we usually we take off the -u and add -anai, but in this case we'd end up with a double vowel, ka-anai. So we must add the わ for smoothness)
please don't buy: かわないでください
Please don't buy that shirt. It's scary.