Because is more common than as and since, both in writing and speaking. When we use because, we are focusing on the reason.

Because強(qiáng)調(diào)原因，更常用。

We often use as and since when we want to focus more on the result than the reason. As and since are more formal than because. We usually put a comma before since after the main clause:

As 和 since 關(guān)注結(jié)果勝過(guò)于原因。

Since 從句里一般會(huì)加逗號(hào)分隔。

當(dāng)because , as ,since 在句首的時(shí)候，從句要用逗號(hào)分隔。

We use because, not as or since, in questions where the speaker proposes a reason:

Are you feeling unwell because you ate too much?

在疑問(wèn)句中，只能用because