while、when、as的用法區別：while常表示一個較長的動作,它引導的從句動作與主句的動作是只能是同時發生的、是平行的;when可以表示較短的動作也可以表示較長的動作,主句和從句的動作可以同時發生也可以先后發生;as與上兩詞同義,可替換while和when, 表示主句和從句的動作同時發生,常譯為一邊一邊。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與nottill/until的區別：前者表示一個延續性的動作,后者表示一個才開始的動作。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與until基本可以互換,但是在句首時只能用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.