however(然而)

　　The text is difficult. It is ， however， not beyond the reach of the students.

　　At first the girl refused to go with him. Later， however， she changed her mind.

　　for(因为)

　　You’d better put on your coat， for it’s rather cold outside.

　　It must be snowing， for it is so bright outside.

　　The old man sat in front of the fire most of the day， for it was very cold