为什么用by playing 而不用through playing

They often play basketball after school.为什么用play而不用playing, the cat is playing with ( ) own tail 为什么用its而不用itself playing 英语,为什么53用through而不用across Playing sports is good for us 这里为什么用playing Did you hurt yourself playing basketball?为什么不用play basketbal he will have been playing on the piano by that time 为什么要用on Does he like后为什么用playing time playing games 为什么用ing形式 为什么要用“across the room"而不用"through the room"? 为什么要用through the telescope而不用with the telescope 穿过沙漠为什么多数与through而不用across?