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.