"I have a big family, so I should buy a lot of water."
Translation:Mam dużą rodzinę, więc powinienem kupować dużo wody.
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The problem is, that often both versions can be a translation of the English sentence, depending on the interpretation. Often one of them seems more probable, but still, the other works. Here, both seem quite probable. Maybe you live with your big family, so you should "kupować" (buy regularly) a lot of water. Maybe your big family is coming for this weekend, so you should "kupić" (buy once) a lot of water so they have enough to drink during this weekend.