It is one of the ugliest faces (that) I have ever seen.这是我见过的最丑陋的头像之一。（that在关系从句中作宾语，可省略）

There aren't many students who sing as well as she does.在学生当中没几个人唱歌像她那么好。（who在关系从句中作主语，不可省略）

People are not so honest as they once were.人们不再像以前那样诚实了。（not so/ as…as用于比较状语从句）

He had hardly had time to settle down when he sold the house.他还没等安顿下来就卖掉了房子。（hardly…when用于过去完成时）

No sooner had I sat down than he came in.我刚坐下他就进来了。（no sooner…than用于过去完成时；否定词位于句首时句子要倒装）

The box was so heavy that she couldn't lift it.箱子太重了，她搬不起来。（so 形容词 that表示“如此……以至于”）

The thief got such a fright that he dropped the bag.那个小偷吓得把提包都扔了。（such（a） 名词 that表示“如此……以至于”）