I was wondering what the difference was between the polish past perfect and past imperfect tenses, as they were not explained in the course. Thanks for your help!
Well, there's not much to say. You use perfective verbs in Past Perfect, and imperfective verbs in Past Imperfect. We don't consider them to be separate tenses.
So obviously, the 'perfect' version will focus on the result, and 'imperfect' on the process (also: result unknown or unimportant).
"Co wczoraj robiłeś?" (What were you doing yesterday?)
"Co wczoraj zrobiłeś?" (What did you do - and finish - yesterday?)