Learn Polish: Czemu on sam gromadzi zapasy? - Why is he hoarding supplies by himself?

Czemu and dlaczego both ask for a cause or reason (why is he doing this?). Czemu is just a slightly more everyday, conversational version of dlaczego. If you used po co instead, you would specifically be asking about his end goal or purpose (what does he need the supplies for?).

Does sam mean 'same' or 'alone' in this sentence?

In this context, sam means 'alone' or 'by himself'. While it can mean 'same' in phrases like ten sam (the same one), here it describes how the subject on is doing the action. If a woman were hoarding supplies by herself, it would change to the feminine form: ona sama.

What case is zapasy in this sentence?

It is in the accusative case because it is the direct object of the verb gromadzi (he is hoarding what?). The singular dictionary form is zapas (a supply). Because it is an inanimate masculine noun, its accusative plural form looks exactly the same as its nominative plural form.

Why is the imperfective verb gromadzi used here?

We use the imperfective gromadzi because the hoarding is an ongoing process right now. If we used the perfective form zgromadzi, the sentence would jump to the future tense and focus on the completed result (why will he have gathered the supplies?).