Here's a rule with no exceptions: in a negative sentence, the object is always partitive.

Affirmative	Negative
Ostan kirjan. — I'll buy the book.	En osta kirjaa. — I won't buy the book.
Syön omenan. — I'll eat the apple.	En syö omenaa. — I won't eat the apple.
Marcos juo kahvia.	Marcos ei juo kahvia.

It's the same idea as Minulla ei ole autoa from Unit 5: if something doesn't happen, nothing is completed — so there can't be a “whole” object.

That single rule alone will make you sound more Finnish.