The face that she saw was ____ of an old man 为什么填THAT

可以先不看that she saw,因为这个是the face的定语.实际上这个句子结构就是the face was the face,of an old man是修饰后面的这个the face的,但是因为句子前面已经提到过the face,为了避免重复,所以用that代替后面的这个.