连词主要有：when as while before after until as soon as since等
（1）区别when, as, while的用法
①.when当......时候引导的时间状语从句，其动词既可以是延续性动作的动词，也可以是瞬间性动作的动词,如：
_________ the students heard the teachers footsteps, they all stopped talking.
当学生们听到老师的脚步声时，他们都停止了讲话。（从句动作发生在前）
② as当.......时候或一边.......一边引导的时间状语从句其动词的动作是延续性的，而且侧重表示主句和从句的动作并相发生：
We were having breakfast __________ she was combing her hair.
她梳头时我们在吃早饭。
③ while表示在某一段时间里或在期间，其所引导的从句的动作是延续性的，并侧重表示和主句的动作同时发生。如：
I can learn ________ I work.
我可以边工作边学习。
用when, as, while填空：
________ you came in, I was talking with a few boys.
It started to rain __________ we were playing outdoors.
They talked _________ they walked down the river.
__________ it was raining, the students were playing basketball.
Her brother watched TV__________ he ate supper.
(4) before引导的时间状语从句，指时间上的在之前
Ill be back _______________________________________________.
你离开之前我就会回来。
(5) as soon as表示一就，做题要注意主将从现
As soon as we __________ (get) there, he ___________ (tell) us the news.
我们一到那，他就会告诉我们那个消息。
(7) since引导的时间状语从句，从句的谓语动词通常为一般过去时，主句的谓语动词通常为现在完成时
We __________ never __________ (meet) since we ___________ (graduate) from the college.
大学毕业后我们就再没见过面。
(8) till/until引导的时间状语从句 till和until同义，作直到时(为止)解，till多用于非正式文体，until多用于句首；not until通常与after进行转换。如：
I wont go with you until(till) I finished my homework.
=____________________________________________________________________.
等我做完作业我才和你一起去。