"I cannot buy everything."
Translation:Nie mogę kupować wszystkiego.
"Nie mogę kupić wszystkiego" = I cannot buy everything right now. Quite possibly, you are not able to buy at once everything that you were supposed to buy.
"Nie mogę kupować wszystkiego" = I cannot buy everything - regularly, from time to time, etc. Like when your wife wants new jewellery every week, and you just cannot buy every single item that Tiffany's adding to their catalogue.
Well, two years further on, I believe I have a reasonable understanding of the difference between Perfective and Imperfective verbs. You can, generally speaking, look up a verb on Wiktionary, and if it has a (Im)perfective partner, it will be listed.
For example, https://en.wiktionary.org/wiki/kupi%C4%87 looks up "kupić" and the line "kupić pf (imperfective kupować)" appears in the entry.
Another example, https://en.wiktionary.org/wiki/pisa%C4%87#Polish looks up "pisać" and the entry shows the line "pisać impf (perfective napisać)".
You can, but it would change the meaning.
umieć - is about learned ability
móc - is about physical ability, or being allowed to do something.
So nie umiem kupować= assumes buying is an ability that other learned, and you did not. (Like choosing right vegetables/meat).
Nie mogę kupować - I am not able to buy ( the circumstances prevent it) I am not allowed to buy. (or I don't allow myself to buy).
also both nie mogę and I cannot are used sometimes to express "I shouldn't".