for用作连词，主要表示理由，用于引起的分句对前面的话进行解释，常用逗号把它和前面的分句分开。如：

She was angry, for she didn’t know French. 她生气了，因为她不懂法语。

He must be out, for there is no light in the room. 他准是出去了，因为屋里没有灯。

He shook his head, for he thought differently. 他摇了摇头，因为他有不同想法。

The days were short, for it was now December. 白天很短，因为这时已经是十二月。

We rarely stay in hotels, for we can’t afford it. 我们很少住旅馆，因为我们住不起。

You needn’t have watered the flowers, for it is going to rain. 你本不必浇那些花，因为就要下雨了。

He took the food eagerly, for he has eaten nothing since dawn. 他狼吞虎咽地吃了起来，因为他从天亮就没吃过东西。

He was busy packing, for he was leaving that night. 他那时正忙着打点行李，因为那天晚上他就要走了。

She does not go out in the winter, for she feels the cold a great deal. 她冬天不出门，因为他特别怕冷。

for表示结果通常不能放句首，也不能单独使用。