Why can't you say "ona harje"? What is the rule about the "si"?
I think it is because there is no action after,if you say "she is playing cards" -> "hraje karty".
But if you just say "she is playing", you have to add "si" it's like reflective verb.
To be confirmed
he is playing / she is playing?
Or it is playing. All these are possible. The Czech sentence does not specify the person.
Is this playing like joking? Games? Instruments? All of the above?