As, because 同 since

As/ since 與because 同樣係連接詞, 亦都解作「因為」, 但分別在於它們的用法和含意。以下係參考一位文法書Practical English Usage 作者Michael Swan的解釋。英文部份係原稿, 中文解釋係我附加上去。


1) as and since

As and since are used when the reason is already known to the listener/ reader, or when it is not the most important part of the sentence. As- and since- clauses often come at the beginning of sentences.
當我地要提及的原因係大家已知, 就會用as and since, 或者當這個原因並非某句子的重點。因此, as and since的 clause 通常會被放在句子的開端。

As it’s raining again, we‘ll have to stay at home.
Since he had not paid his bill, his electricity was cut off.

As- and since-clauses are relatively formal: in an informal style, the same ideas are often expressed with so.

2) because

Because puts more emphasis on the reason, and most often introduces new information which is not known to the listener/ reader.
如果改用Because時, 一般係要去強調所提及之原因, 或者此原因係對方未知的。

Because I was ill for six months, I lost my job.

When the reason is the most important part of the sentence, the because-clause usually comes at the end. It can also stand alone. Since and as cannot be used like this.
由於此原因係句子的重點, 所以通常留在句子的末端。甚至可以用作獨立的句子。

Why am I leaving? I‘m leaving because I‘m fed up!
(NOT…I‘m leaving as/since I‘m fed up!)

Why are you laughing? Because you look so funny