for 和 because用法的分析, 都可以表示“因為”, 如何區別?

問：下面是《大學英語四級考試常見錯誤分析》(p. 33) 對for 和because 的正誤分析:

“誤: The day breaks because the birds are singing.
正: The day breaks for the birds are singing.
譯: 天亮了, 因為鳥在叫。
析: because 表示真正的原因, 因為鳥叫不是天亮的原因, 要日出才能破曉, 所以只能說: The day breaks because the sun is rising. 鳥叫不是天亮的原因, 而是結果。
鳥之所以叫, 其理由是天亮了。這種倒果為因的說法, 英文只能用for, eg: It is going to rain, for the barometer is falling. (天要下雨了, 因為晴雨計正在下降。)
誤: The river has risen, for it has rained much of late.
正: The river has risen, because it has rained much of late.
譯: 河水漲了, 因為近來下了許多的雨。
析: because 指的是道德的或自然的原因(moral or physical cause), 而for 則指的是邏輯上的理由(logical reason) 。請看下例: It must have rained much of late, for the river is so high.(近來一定下了許多雨, 因為河水漲得這樣高。) 這句談的是理由。”

下面是《英語疑難大全》(天津大學出版社, p. 534) 對for 和because 用法的分析:

“如果子句並不表示原因, 而只是為主句中所說的話提供推斷的理由加以解釋時, 子句只可用for 引導, 且for 前常有逗號。
誤: He must be at home because (as, since) the light in his room is on.
正: He must be at home, for the light in his room is on.
房子裡的燈是開著的, 並不是他在家的原因, 而只是為證明他在家提供的一個推斷的理由。”

讀了上面的說明, 我產生一個問題: 難道for 只能用來表示“由for 子句所表示的結果推斷出前一個子句所表示的原因”嗎? for 子句難道就不能表示原因嗎? 如果不能表示原因, 那如何解釋下面的句子?
The days were short, for it was now December.
能否說for it was now December 是結果, 而the days were short 是原因呢?

答：以上兩本書清楚地解釋了for 的一種用法以及它與because 的區別。上面的講法對於英語初學者是適當的。但是, 英語水準較高的讀者應進一步瞭解: 連接詞for 也可以表示原因或理由, 請看下面的實例:

1. She’s gone for a walk, for the rain has stopped. 她出去散步去了, 因為雨停了。
2. She’s gone for a walk, because the rain has stopped. 她出去散步去了, 因為雨停了。
3. Even in his irritability he was gentle, for Leora was with child. 甚至在他煩躁時, 他也顯得很溫和, 因為利奧拉懷孕了。
4. She was angry, for she didn’t know French here for is correct; because is also possible). 她生氣了, 因為她不懂法語。

小練習：請選出正確的答案。(留言作答)
Someone must have left the tap on _________, the water was running over and flooding the bedroom.
A. therefore
B. for
C. nevertheless
D. moreover
(攻讀碩士學位研究生入學考試英語試題, 84 A)

另見for用法 和 due to用法