日本語 Japanese Ask Questions Thread 4

If you have a question about the language, ask it and fellow 4channers might see it and answer it for you.


Japanese Ask Questions Thread2
http://dis.4chan.org/read/lang/1206158123/1-40

Japanese Ask Questions Thread3
http://dis.4chan.org/read/lang/1267485093/1-40

This is a  MUCH more noobish question then everyone elses, but I'm looking over my japanese review and am for some reason totally spacing out on this one.

It's a correction question, where the first sentence is wrong and needs to be corrected.
First sentence:
きのう、デパートで何も買いました。
Second sentence:
きのう、デパートで何も買いませんでした。

My derp question is, what is the form of　かう in the second sentence?
And why must it be changed to that form?