概念
在复合句中，由时间状语从句表示主句动作发生的时间。(在复合句中，要注意主句和从句的时态大多都要保持一致。）
要点
时间状语从句，由以下连词引导：
when while as after before as soon as since till /until by the time 在时间状语从句中，要注意时态一致。一般情况下主句是将来时的时候，从句主要用一般现在时。
when在...的时候
Mozart started writing music when he was four years old.
(在)莫扎特4岁的时候，开始写音乐作品。
while在...期间
He visited a lot of places while he was traveling.
他在旅行期间参观了许多地方。
as在...的同时；一边...一边..；随着
We alwalys sing as we walk.
我们总是一边走一边唱。
I'll let him know as soon as she comes.
她一来，我就会让他知道。
after在...之后
He left the classroom after he had finished his homework .
他做完作业之后就离开教室。
before 在...之前
Mr. Brown had worked in a bank for a year before he came here.
布朗先生来这里之前已经在一家银行里工作一年了。
as soon as 一。。。就。。
We began to work as soon as we got there.
我们一到那就开始工作。
I will write to you as soon as I get home.
我一到家就给你写信。
since 自。。。以来 到现在
表示自过去的一个起始时间点到目前(说话时间)为止的一段持续时间。主句一般用现在完成时，从句用一般过去时。还可以用作介词，本句从句还可以用短语：since three years ago(从三年前至今)表示。
Mr Green has taught in that school since he came to China three years ago.
自格林先生来中国以来，他就在这所学校教书。
till /until直到。。
都可以作连词，连接时间状语，也可以作介词，与其它词构成介词短语，在句中作状语。（强调将一般用until）
They walked till /until it was dark.
他们一直走到天黑。
Xiao Ming didn’t leave home till / until his father came back.
小明直到他爸爸回来才离开家。
by the time 到。。。为止
(所在句子的主句应用完成时)
By the time he gets there , his father has already gone.
他到家的时候，他爸爸已经走了。
By the time I got to school, the class had already begun.
我到校时，已经开始上课了。
