二.原因状语从句
I) because, since,和as引导的原因状语从句
1. 其中because语气最强，多表示所述理由是本句重点，强调别人未知的事实，故because引导的从句多放在句末，回答why的问句时，只能用because。
在语气上since次之，as最弱。since和as表示的原因，多是已知的既成事实，原因以外的才是叙述的重点，因而两者多用于句首。
As it is raining, lets stay indoors.
Since you have no license, you are not allowed to drive.
用强调句式强调原因状语从句时，只能用because引导。
It was because he was careless that he failed.
在because引导的从句中，如果主句是否定句，而主句和后面的从句之间没有逗号分开，这时not 修饰的是后面的整个从句。
The machine did not stop because the fuel was used up.
机器停了下来，并不是因为燃料用光了。
如果主句和从句间用逗号分开，意思就不同了。
I was not kind to him, because he was rude. 我对他不友好，因为他无礼。
II) for 和because 的区别
because 表示因果关系，语气最强;而for引导的是并列句，它只是对前面分句进行解释， 说明推断的理由，并且只能后置。
It must have rained last night, for the ground is wet this morning.
It is morning, for the birds are singing. 天亮了， 鸟儿在鸣叫。