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 teacher's 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"

I'll 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 won't go with you until (till) I finished my homework. =_________

等我做完作业我才和你一起去。