Hi Máté,
I'm aware that we've been moving very slowly (which is completely my fault!), so once you've read that lesson I want to move on asap whilst I've got the chance to do some stuff with you. Have a look at this
incomplete piano sonata by Mozart. If you can give it play through.
Now whenever you look at any piece of tonal music, as I'm sure you are, you should always ask yourself what key it's in. I think the title gives this one away a bit... C major.
Ok so now ask yourself: does it always stay in C major?
What happens bar 21? Where do those f sharp come from? Why are they there (i.e. what purpose do they serve)?