hang back
phrasal verbhang back 
- to remain in a place after all the other people have left留下;继续留在原处 
hang back (from something) 
- to hesitate because you are nervous about doing or saying something犹豫;畏缩;吞吞吐吐 - I was sure she knew the answer but for some reason she hung back.我敢保证她知道答案,但不知为什么她不敢说出来。 
 
- I was sure she knew the answer but for some reason she hung back.