21.9 had better表示比較好

　　had better 相當(dāng)于一個(gè)助動(dòng)詞，它只有一種形式，它后面要跟動(dòng)詞原形。
　　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表示與事實(shí)相反的結(jié)果，意為"本來最好"。
　　You had better have come earlier.