"What are they playing?"
I got this right with "Na co hrají." So I'm wondering, using the music sceanrio...
Would "Na co hrají" mean, for example, "What instruments are they playing?" while "Co hrají" might mean "What music are they playing?" Or did I just get lucky, and there's no difference between the sentences with and without "na"? :-)
When do I use hrajou si and when do I use just hrajou?
More or less any verb that is both reflexive and non reflexive means, when used as a reflexive one, that the subject of the sentence does the thing for themselves. HRÁT SI means in most cases that they are playing with toys. They are entertaining themselves. You can also HRÁT SI music, but it would only be for yourself or yourselves. Once you play music for audience, you just HRÁT. The same goes for games, soccer or cards, board games. You HRÁT chess, etc.