Learn Italian: Conoscevo Marco, ma non sapevo dove lavorava. - I knew Marco, but I did not know where he worked.

In Italian, conoscere is used for being acquainted with people, places, or things. Therefore, you use conoscevo for Marco. Sapere is used for knowing facts or pieces of information. Here, you use sapevo because where he worked is a factual piece of information.

Why are all the verbs in this sentence in the imperfetto tense instead of the passato prossimo?

The imperfetto is used to describe ongoing states or situations in the past. Knowing Marco, not knowing a fact, and his employment were all continuous, ongoing states in the past, rather than single, completed events.

What would it mean if I used the passato prossimo forms here, like ho conosciuto or ho saputo?

The meaning would change completely! Ho conosciuto Marco translates to I met Marco, describing the exact moment you were introduced. Ho saputo means I found out or I realized. The imperfetto describes the ongoing state of knowing, not the sudden moment of discovery.

How do we know lavorava means he worked and not I worked?

In the imperfetto tense, the I form ends in -o (lavoravo), while the he/she/it form ends in -a (lavorava). While lavorava could technically mean she worked or formal you worked, we know it means he worked here because Marco is the only logical subject from the first half of the sentence.