英语原因状语从句的用法大全
一、往期状语从句回顾
二、原因状语从句
常用来引导原因状语从句的从属连词有：because、as、since、for。
除此之外，还有一些其实也较为常见但对于不少基础阶段的英语学习者来说不太熟悉的连接词语，如：now (that)、when、seeing (that) 、considering (that)、given (that)、in that、for the reason that等。
（一）because、as、since、for的用法
这四个连接词是最为常见的，也比较容易混淆，所以经常在各种英语考试中考查到。下面我们就来梳理梳理。
1、because的用法
because引导的原因状语从句一般放于主句的后面。当because从句位于句首时，要用逗号分开；放在句末时，可不用逗号分开。如：
从句在后（有逗号）：We didn't go for an outing last Saturday, because it was raining hard the whole day.
从句在后（无逗号）：We didn't go for an outing last Saturday because it was raining hard the whole day.
从句在前（必须有逗号）：Because it was raining hard the whole day, we didn't go for an outing last Saturday.
because表示直接原因， 在这组连接词中语气最强， 一般用来回答why引导的疑问句。如：
-Why didn't you come to my birthday party last night?
- Because my sister was ill. I had to take care of her the whole night.
【注意】
1）与汉语不同的是，英语中的because 不能与so连用。比如：
错误：Because it was raining hard the whole day, sowe didn't go for an outing last Saturday.
正确：Because it was raining hard the whole day, we didn't go for an outing last Saturday.
2）because of 也表示原因，但与从属连词because不同的是，它是介词短语，后面不接从句, 只能接名词、代词或动名词及其短语。如：
He can't go to school because of his illness.
He can't go to school because he is ill.
3）not...because结构
该结构中的否定词有时否定主句，有时否定从句，一般要根据句子的意思作出正确或合乎逻辑的理解。若not否定主句，最好在because之前用逗号，否则会引起歧义。如：
I didn't go there yesterday because I was afraid.
在没有上下文的情况下，这句话有两种理解，即：
否定主句：I didn't go there yesterday, because I was afraid. （我因为害怕，所以昨天没有去那儿）
否定从句：I went there yesterday, not because I was afraid. （我昨天去那儿，并不是因为我害怕。）
不过，如果because之前有just修饰，一般认为not 是否定从句的。如：
You shouldn't get angry just because some people speak ill of you. （你不要因为有人说你坏话而生气。）
从这个意义上讲，《新概念英语》第二册第29课的最后一句如果没有上下文也会引起歧义：
...but Captain Faucett did not take him because the trip was too dangerous.
以上句子就可以作两种理解，即：
否定主句...but Captain Faucett did not take him, because the trip was too dangerous.
否定从句：...but Captain Faucett took him, not because the trip was too dangerous.
2、since的用法
since引导的原因状语从句一般放于主句之前，表示双方都已知的、显然的理由，通常被翻译成“既然”，相当于now that。since的用法较为正式， 语气比because弱。
Since you are free today, can you help me with the housework?
Now that you are grown up, you should rely on yourself.
【注意】
since也常后接时间或过去发生的事情，来表示“自从...以来”，其引导的从句动作一般都采用一般过去时，而其主句经常使用完成时。如：
I have lived here since 2015. （我从2015年开始就住在这儿了。）
She has learned English since she was two years old. （她从两岁开始就学习英语了。）
3、as的用法
as引导原因状语从句时，表示附带说明的“双方已知的原因”，含有对比说明的意味，语气比since弱， 较为正式。位置较为灵活，但常放于主句之前。
As the wage of the job was low, there were few applicants for it.
As you are tired, you had better take a rest.
【注意】
as常用来引导时间状语从句和方式状语从句。如：
时间状语从句：A sudden chill of horror sweeps over her as she feels the drip of saliva upon her hand. （当她感觉到滴到她双手上的唾液时，恐怖的寒栗突然遍布了她全身。）
方式状语从句：She did it as I had told her to.
4、for的用法
for引导的是并列句，表示原因，但并不构成主句行为发生的直接原因，只提供一些辅助性的补充说明。for引导的并列句只能放于主句之后，并且必须用逗号将其与主句隔开。如：
It must have rained last night, for the street is wet all over.
主句猜测昨天晚上肯定下雨了，理由是街道都湿了。但是，街道上湿了，不一定是下雨了，也有可能是洒水车洒的，所以主从句不构成直接的因果关系。
在《新概念英语》第二册第43课里，也有一句话是用了for来引导表原因的并列句的，即
Byrd now knew that he would be able to reach the South Pole which was 300 miles away, for there were no more mountains in sight.
句中的for引导的句子，补充说明为什么Byrd知道他们能够安全抵达南极的原因，即前面再没有山了。
顺便说一句，该句其实有个纰漏，即句中的定语从句是限定性定语从句，但考虑到the South Pole具有唯一性，所以应该使用非限定性定语从句才对，即要在Pole后面加上逗号。
（二）其他连接词语的用法
其他常见的用来连接原因状语从句的连接词语有：now (that)、when、seeing (that) 、considering (that)、given (that)、in that、for the reason that等。如：
She is much more mobile now that she has a car. （她现在出去方便多了，她有了一辆汽车。）
I won't tell you when you won't listen. （既然你不想听，那我就不告诉你了。）
He looks young considering his age. （照他的年纪看来，他显得年轻。）
Given that he was still a boy, I forgave him. （考虑到他还是个孩子,我就原谅他了。）
We aren't going for the simple reason that we can't afford it. （我们不去，只是因为我们负担不起。）