Then she leapt backwards, back arched and fur on end ，tail held out stiffly（脊背拱起，毛发竖立，尾巴僵直地伸着）.
2. as if 引导表语从句
It looked as if （看起来好像）someone had cut a patch out of the air. . .
3. If so 状语从句的省略
If so（如果这样的话），what did you like best?
4. be doing sth. . . . when正在做某事，突然……
A young man was walking through a wood when （正穿过一片树林这时） he saw a ring lying on the ground.