原因状语从句
连词有：because as since now that等
（1） 区分because, as, since, for
① because表示人们不知道的直接的原因或理由，着重点在从句，其所引导的从句，为全句句意的中心所在。通常用于回答why引出的疑问句，语气最强，除特别强调外，该从句一般位于主句后面，不能与so连用但是可以与so进行同义句转换。
I didnt go aboard with her because I couldnt afford it.
我没有和她一起出国是因为费用太高。
② as引导的原因状语从句所传递的通常是已知信息，从句多位于主句之前，表示双方那个已知的理由，如：
As I didnt know the way，I asked a policeman.
我不认识路，因而问警察。
③ since引导的原因状语从句多表示大家众所周知的理由，如：
Since traveling by air is much faster, they decided to take a plane.
既然乘飞机旅行快得多，他们就决定坐飞机。
④ for表示的理由是附加的或推断的，并非此原因，通常与主句用逗号隔开，不位于句首，如：
He must be not at home, for the telephone isnt answered.
用because, as, since, for 填空：
Well have to go to bed early ___________ well start off early tomorrow.
___________ you know all about it, tell me please.
____________ all the seats were taken, I had to stand.
He must be at work, __________ the light in his office is still on.
Lily didnt go to school ___________ she was ill.
=_________________________________________________________.
(4) now that引导的原因状语从句
Now that 因为；既然，通常可以和since换用。用来表示一种新的情况，再加以推论。如：
Now(that) you have passed your test, you can drive on your own.
你既已考试合格，就可以独自开车了。