表示因果关系的连词

可表示“因为”，但引导的不是从句，而是分句，对前面情况加以解释，常用逗号把它和前面的分句分开，这在书面语中比较多见（口语中多用because, as, since等）：
The days were short, for it was now December. 白天很短，因为现在已经是十二月。
He took the food eagerly, for he had eaten nothing since dawn. 他吃得很香，因为白天亮起他就没吃东西。
The children soon lost their way, for they had never been in the forest before. 孩子们很快迷了路，因为他们从未进过这座森林。
它还可以表示为什么有前面的看法（此时不能用because, as, since）：
It must have rained in the night for when I woke the next morning I saw the grass wet. 夜里准是下雨,因为我早上我醒来看见草地是湿的。
She must have gone out early, for she had not shown up at breakfast. 她肯定一早出去了，因为她没来吃早饭。

表示结果，可译为“因此”、“所以”：
Nobody seemed about, so I went in. 附近似乎没人，因此我走了进去。
The play began at eight, so they must dine at seven. 我八点开始，所以他们得七点吃饭。
She felt very tired so she went to bed early. 她感到很疲倦，于是早早就睡觉了。
Our cases were heavy, so we took a taxi. 我们的箱子很重，因此我们坐了出租车。

副词，也表示同样的意思，但比较文气一点，可以放在分句前：
There is fog at Heathrow; therefore the plane has been diverted. 希斯罗机场有雾，因此飞机改到别的机场降落。
It rained; therefore the game was called off. 天下雨了，比赛因此取消。
也可插在句子中间：
He's out of the country and therefore unable to attend the meeting. 他在国外，因此不能参加会议。
He had finished the Times, there was therefore nothing to do. 他已看完泰晤士报，因此没事可干了。
He had gone; she therefore gave the money to me. 他走了，于是她把钱给了我。

也是副词，也表示类似意思，多用在说理性文章中，且多用在分句或句子开头：
My mother is by herself; hence I must go home now.  我妈一个人在家，因此我得回去了。
Microwaves have got cheaper and hence more people can afford it. 微波炉便宜些了，因此更多的人买得起了。
Hence he was annoyed by many unwelcome attentions. 因此许多不受欢迎的殷勤举动惹得他不高兴。
有时后面只跟一个动词：
The town was built on the side of a hill, hence the name Hillside. 这座城市建在山腰，因此称作山腰城。
I fell off my bike yesterday—hence the bruises. 我昨天从自行车上摔下来了，因此身上摔青了。