英语语法问题Awoodfirewasburningonthehearth,andacatwassleepinginfrontofit.Agirlwasplayingthepianoandsingingsoftlytoherself.Suddenlytherewasaknockonthedoor.Thegirlstoppedplaying.Thecatwokeup.就是想问一

　　英语语法问题

　　Awoodfirewasburningonthehearth,andacatwassleepinginfrontofit.Agirlwasplayingthepianoandsingingsoftlytoherself.Suddenlytherewasaknockonthedoor.Thegirlstoppedplaying.Thecatwokeup.

　　就是想问一下最后这两句,为什么一个过去进行时一个一般过去式?有没有简单的方方法来区分这两种时态?