Quote Originally Posted by Wyrm
Я не покупаю книгу вчера = I did not buy the book yesterday (according to google)
As already was said it is incorrect.

It has a perfective prefix with a regularly conjugated suffix. Isn't that pefective future tense?
No, "покупаю" is imperfective present (1st person, single). Prefix does not guarantee perfection...

купить - покупать (perfective - imperfective)

I think when there is suffix а in a verb and it is stressed, the verb should be imperfective no matter of prefixes.

Я не купил вчера книгу. I did not buy the book yesterday. (perf)
Я не покупал вчера книгу. I did not buy any book yesterday. (imperf.)
Я не покупал вчера книг. I did not buy any book yesterday. (imperf.)