@victorgijsbers It wouldn't sound horrible to keep playing g minor, but it might be boring which is why the sonata is mixing things up in the second section. (Both f and f# are a part of g minor.) The sharp is because the next chord is A, which leads us to the Dm that follows (and which is very closely related to the Gm). Then we get the E leading to Am, Dm, G, C but instead of landing there the C turns into C7 and moves us to F for the restatement of the main theme.