21.9 had better表示比较好

　　had better 相当于一个助动词，它只有一种形式，它后面要跟动词原形。
　　had better do sth
　　had better not do sth
　　It is pretty cold. You'd better put on my coat.
　　She'd better not play with the dog.

　　had better have done sth表示与事实相反的结果，意为"本来最好"。
　　You had better have come earlier.