while、when、as的用法区别：while常表示一个较长的动作,它引导的从句动作与主句的动作是只能是同时发生的、是平行的;when可以表示较短的动作也可以表示较长的动作,主句和从句的动作可以同时发生也可以先后发生;as与上两词同义,可替换while和when, 表示主句和从句的动作同时发生,常译为一边一边。如：Please do not trouble me while I am writing my homework./ Ill go home when I have finished my job./ They were running quickly across the road when they heard the sound of a truck coming./ As we walked in the dark street, we sang songs and talked loudly.
till/until与nottill/until的区别：前者表示一个延续性的动作,后者表示一个才开始的动作。如：I will stay here and watch the baby until you return. / They wont go on working until they get what they think is reasonable.
另外till与until基本可以互换,但是在句首时只能用until,不能用till. 如：Until the last minute of the match we kept playing./ Not until he had finished his work did he go home.