在英语中，单词 “because、as” 和 “since” 都可以用来说明一件事发生的原因，但是这三个单词强调的重点有细微的差别。哪个词语引导的从句着重强调原因？哪个更注重说明一件事的结果？在使用这三个词的时候，有哪些值得注意的地方？
 
Hi, I’m Tim from BBC Learning English. Today I'm going to tell you about three useful words: because, as, and since.
These are all used to give reasons.
Because is more common than as or since and we use it when we want to focus on the reason.
I'm tired because I didn't sleep very well.
When we want to give extra focus to the reason, we can put the because-clause at the beginning of the sentence.
Because my bed is uncomfortable, I'm getting a bad back.
As and since are more formal than because and we use them to focus on the result rather than the reason.
I hope Tom's brought that comic as I wanted to borrow it from him.
Noodles are popular, since they're easy to cook.
We often use as and since clauses at the beginning of a sentence. Just remember to add a comma.
So, as our minute is up, I'll finish recording this video.
 
用法总结
1 “Because” 比 “as” 和 “since” 更常见。我们用 “because” 来强调事情的原因。
I'm tired because I didn't sleep very well.
如果想要额外强调原因，可以将 “because" 引导的从句放在句首。
Because my bed is uncomfortable, I'm getting a bad back.
 
2 “As” 和 “since” 比 “because” 更为正式。我们用 “as” 和 “since” 来强调事情的结果。
I hope Tom's brought that comic as I wanted to borrow it from him.
Noodles are popular, since they're easy to cook.
“As” 和 “since” 引导的从句也常用于句首，注意不要忘记在从句后加逗号。
So, as our minute is up, I'll finish recording this video.