As, because 和since都是连词，都可以引导原因状语从句，表达产生结果的原因。
|He decided to go to the conference in Barcelona,||as he was in Spain anyway.|
|Are you angry with me||because I opened the letter?|
|Bilardo coaches his team by telephone,||since half of them play in Italy, France or Spain.|
She spoke quietly because she didn’t want Catherine to hear.
We’ll come over on Sunday because David’s got to work on Saturday.
Because breathing is something we do automatically, we rarely think about it.
We can use a because-clause on its own without the main clause in speaking or informal writing:
A: Would you like to go to school there?
B: Because my best friend goes there. (I would like to go to school there because my best friend goes there.)
In 1998, the government introduced a new import tax because people were importing cars from abroad.
在非正规的书面语或口语中，我们会把because缩减为Cos /kəz/ 或 /kɒz/ 。
I’m laughing cos I’m so happy.
As and since
当我们更关心结果，而非原因时，我们会用as和since, as和since比because要正规。通常在主句在前，然后有个逗号，接着 since引导的时间状语从句在后。
[结果]I hope they’ve decided to come as [原因]I wanted to hear about their India trip.
[结果]They’re rather expensive, since [原因]they’re quite hard to find.
Since everything can be done from home with computers and telephones, there’s no need to dress up for work any more.
As everyone already knows each other, there’s no need for introductions. We’ll get straight into the business of the meeting.
Are you feeling unwell because you ate too much?
Are you feeling unwell since you ate too much? or … as you ate too much?
Since you ate the ice cream last night, we don't have any dessert tonight.
He has been stronger since he joined the army.
As it was raining hard, the children were unable to play football.
我们不说：The children stopped playing football as it started to rain。
在此句中The children stopped playing football as it started to rain的as很可能被误解为："The children stopped playing football at the time it started to rain." 所以不建议这样使用。这就是为上面as表示原因一般放在开头，如：As it was raining hard, the children were unable to play football.
|because||更强调原因||The students could go home earlier because the teacher was ill.|
Because the teacher was ill, the students could go home earlier.
|as||原因显而易见.||As we read, we learn.|
|since||原因已知. (比 as正规)||Since we were in the computer lab, our English has improved.|
|for||原因放在最后，不能把 for 放在句子开头||We went to a small restaurant - for we were hungry.|