Learn Polish: Dlaczego jesz zimną konserwę zamiast obiadu? - Why are you eating cold canned food instead of dinner?

Dlaczego asks for the reason or cause behind an action (why did this happen?), while po co asks for the purpose or goal (what is the intended result?). Since the speaker is asking what circumstances led to you eating canned food rather than what you hope to achieve by doing it, dlaczego is the natural choice.

Why is the word for dinner spelled obiadu here?

The word follows the preposition zamiast (instead of), which always requires the genitive case. For masculine inanimate nouns like obiad, the genitive singular usually takes the -u ending, making it obiadu.

What case is zimną konserwę in?

It is in the accusative case because it is the direct object of the verb jesz (you are eating). For feminine nouns, the nominative zimna konserwa changes its endings in the accusative: the adjective gets -ą and the noun gets -ę.

Does konserwa mean fruit preserves or jam?

No, it is a bit of a false friend! Konserwa refers to savory canned food, like canned meat or fish. If you wanted to talk about sweet fruit preserves, you would use words like dżem or konfitura.