was/ were doing...when...
I

was sitting in my room

was sitting in my room

with my cat，Smartie，on my lap，

when

when

the roof just flew off.
我正坐在房間里，我的貓咪斯瑪?shù)倥吭谖业耐壬?，這時(shí)屋頂就飛走了。