while是什么時(shí)態(tài)的

　　例句：

　　Carry on with your work while I'm away.

　　我不在時(shí)你要接著干。

　　I waited while he gathered up his papers.

　　他整理文件時(shí)我就在一旁等待。

　　They palled up while they were at college.

　　他們上大學(xué)時(shí)成了朋友。