Ok, for starters, I do know what the difference is but only "in theory." I am well aware that perfective implies the consequence (or completion) as well as the time of the verb while imperfective implies it is in the process of occurring, is being repeated, or has not happened yet.

However, a lot of the time I do not what the heck this means. For examples, I will use http://www.alphadictionary.com/rusgrammar/aspect.html and their examples.

"Завтра я буду писать весь день."
"Tomorrow, I will be writing all day"

This is imperfective future. This makes sense I suppose. The writing has not happened yet; the letters have not been written. He will be writing all day tomorrow.

HOWEVER,

"Вчера Петя пошёл в библиотеку в семь часов"
"Yesterday, Peter went to the library at seven o'clock"

According to my textbook, this is imperfective past. This is an event that has been completed, involved time, and is a one-time occurrence. I have no earthly idea how this makes sense to put it under imperfective. It follows the criteria for almost every aspect of perfective. Why isn't this perfective?!!?

I feel like once I grasp this concept, my life will be much easier