Du is the second person SINGULAR, ni is the second person PLURAL. So depending if the subject you're using is one or more humans you use du or ni. Du pratar svenska (you, singular) Ni pratar svenska (you, plural)

What Elisa said! :) "Ni" can also be used instead of "du", but only in formal situations. I don't think it's very common nowadays.

du - você ni - vocês Fácil 😶