"I need to buy paint."
Translation:Мені треба купити фарбу.
I am not Ukrainian, but to my Slavic ear, "Kupuvati" would mean "to be buying" (indicative form), which would roughly translate to "I need to be buying paint" .
It would mean that you would need to be buying paint every day, because it's for example, necessary for your job, rather than to buy it once in order to e.g. paint your fence :-)