because，for，since 與 as 的用法
表示“原因”的這幾個連詞，從語氣上由強至弱依次為
becauses→since→as→for
其中，because，since，as是從屬連詞，引導原因狀語從句
for是並列連詞
1、because表示直接原因，可用來回答why引導的問句，一般放在主句之後，也可以單獨存在
We stayed at home because it rained.
因為下雨我們待在家裡了。
— Why didn't you come to the party?
你為什麼沒參加聚會？
— Because my mother was ill.
因為我媽媽病了。
2、since意為“因為；既然”，則側重主句，從句表示顯然的或已知的理由
Since his computer doesn't work, let's try yours.
因為他的電腦壞了，讓我們試試你的吧。
Since everyone is here, let's begin our meeting.
既然大家都到了，我們開會吧。
As it is cold outside, we'll put on our overcoats.
因為外面冷，所以我們要穿外套。
As it was late,I made haste to go.
由於晚了，我得趕緊走了。
4、for意為“因為；由於”表明附加或推斷的理由，引導的分句前常又逗號，for分句不放句首
He must be ill, for he is absent today.
他一定病了，因為他今天缺席了。
It rained last night, for the ground is wet.
昨夜下雨了，因為地面潮溼。
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