Breakdown of Po co kupiłeś taki duży zapas wody?
duży
big
woda
water
kupić
to buy (perfective)
taki
such, this kind of
Questions & Answers about Po co kupiłeś taki duży zapas wody?
Why use Po co here instead of Dlaczego?
Po co asks for the purpose or goal ("What for?"). The speaker wants to know what the listener plans to do with the water. If they used Dlaczego (Why?), they would be asking for the cause or reason leading up to the purchase, such as "Because there was a sale" or "Because the news said to stock up."
Why is water written as wody here instead of the direct object wodę?
Because the direct object of the verb kupiłeś (you bought) is actually zapas (supply). Wody is in the genitive case because it describes what the supply consists of (a supply of water). If you were buying the water directly without the word zapas, you would use the accusative case: Kupiłeś wodę.
Taki duży zapas is the direct object. Why doesn't it change its ending for the accusative case?
It is in the accusative case, but zapas is a masculine inanimate noun. For singular masculine inanimate nouns (and their adjectives), the accusative form is identical to the nominative (dictionary) form. If it were a masculine animate noun, like a dog (pies), the ending would change (takiego dużego psa).
Could I say tak duży zapas instead of taki duży zapas?