时间
连词：
when（当…时）
主、从句的动作或状态同时、之前、之后发生，主从句的时态；
主将从先
例：I will write to you when I get to Hongkong
主句过去式 从句过去式
例：He worked as a newspaper boy when he left school.
主句现在（过去）进行时 从句一般现在（过去）时
例：My mum was cooking dinner when I got home.
主句过去完成时 从句一般过去时
例：The train had left when I got to the station.
while（ 在…期间， 在…之时）
所引导的从句中的动词或状态是延续性的, 后可以接分词短语；
主句过去进行时 从句过去进行时
例：My mother was cooking while my father was reading newspaper.
主句一般过去时 从句过去进行时
例：I lost my wallet while I was jogging through the park.
as （正当…时，边…边…）
表示主、从句的动作或状态同时或几乎同时发生；
主从句时态相同
例：As he sat down, his mother came in.
主句现在（过去）进行时 从句一般现在（过去）时
例：As he was watching TV, his mother came in.
since (自从…以来）
主句完成时，从句一般过去时
例： It is/has been five years since I began to learn English.= (I have learned English for five years.)
until （直到...）
转换
before
例： I stayed here until my mother came yesterday.
after
例： I won't believe you until I have seen it with my own.
主将从先
例： I won't believe you until I have seen it with my own.
主句与从句时态相同
与延续性动词连用，用肯定句；
例： I stayed here until my mother came yesterday.
与瞬间动词连用，用否定句。
例：I didn’t leave here until my mother came yesterday.
as soon as （ 一… 就 …）
主将从先
例：I will call you as soon as he comes back.
主从句时态相同
例：They rushed out as soon as the class was over.
原因
because
“因为...”语气最强, 表示的是直接理由，通常放句中, 不能同so连用。
转换
because of+短语
例：The ship changed its course because there was a storm.
so＋句子
例：Because he was ill,he didn’t go to school.
主从句时态相同
例：I was late because there was a traffic accident.
since
“既然…..” 表示对方已知的事实或理由，常放在句首, 不能同so连用。
主从句时态相同
例：Since you have enough money, you can come and buy it.
例：Since smoking is harmful to people, you’d better give it up.
as
“由于….” 语气较弱，较口语化，表明显的原因或已知的事实,常放在句首, 不能同so连用。
让步
though
Remember: though and although have the same meaning.
主从句时态相同
例：Though we have grown up, our parents treat us as children.
although
We do not use but together with though or although.
主从句时态相同
例：Although he is rich, he is not happy.
转换
but＋句子
例：He didn’t stop working though he was very tired.
条件
可以主将从现
例：If you work harder,you will succeed.
例：We can’t write to Mary unless she tells us her address.
We can’t write to Mary,if she doesn’t tell us her address.
Work harder, or you won’t succeed.
Work harder,and you will succeed
He was very tired but he didn’t stop working.
例： I stayed here before my mother came yesterday.
I will believe you after I have seen it with my own.
He was ill, so he didn’t go to school.
The ship changed its course because of a storm.
