原因状语从句三个最常用的连词是because, since, as，其中because语气最强,since次
1) because从句通常是直接回答why问句的，这时不能用since, as或for。例如:
Why don't you call on me tomorrow?
Because I have an important meeting to attend.
He could not have seen me, for/because I was not there.
She was not helpless，for/because she had money of her own.
It rained last night，for the ground is wet this morning.
The ground is wet，for/because it rained last night.
It will rain，for the barometer is falling.
The day is going to dawn，for the birds are singing.
As/Since he was not at home，I spoke to his brother.
Since you are as sure of it，he'll believe you.
Since the speaker can't come，we may as well cancel the meeting.
Since the weather is so bad outside，why don't you stay with us overnight?
注意，在强调句it is…that中，只能用because从句，不能用since, as。同时，表示并
列关系的连词，如whereas, while, for也不能用在强调句it is... that。二结构中。
It was because he was ill that he didn't go with us.(正)
，It was since/as/whereas/while he was ill that he didn't go with us.(误)
3)由其他连词或词组引导的原因状语从句，considering (that)(考虑到), in that(因
为),now (that)(既然),seeing (that)(鉴于)，…allowing for...等.例如:
In that he did not come to work yesterday, it showed that he is not very interested
The speech is all the more important in that it was made by a man of authority.
Now that you have finished the work，you are free to do what you like.
Seeing that it is eight o'clock，well wait no longer.
Seeing that all the guests have arrived，let's have dinner.
Considering that he began learning English only a year ago，he speaks very well.
Considering that his wife had just recovered from a bad cold, he decided not to take
her to the party.
Allowing for the train being late，we should be back by eleven.
On the grounds that，for the reason that，on account of the fact that，owing to the
fact that，due to the fact that, in view of the fact等。
She is suing for divorce on the grounds that he is a drunkard.
He cannot be dismissed for the simple reason that he didn't attend yesterday's meet-
On account of the fact that all the trains were delayed，the station was crowded with
We had to anchor in view of the fact the tide ran against us.
5)在not that ... but that结构中，that = because，即not because ... but because.例
I haven't finished writing the report yet, not that I dislike the work, but that I have
He always traveled third-class，not that he was so miserly, but that he wanted to go
side by side with ordinary people.
An extremely selfish person, Peter never cared for other people's interests.
或Extremely selfish，Peter never cared for other people's interests.
或Being an extremely selfish person, Peter never cared for other people's interests.