had better 相當于一個助動詞，它只有一種形式，它后面要跟動詞原形。

　　had better do sth

　　had better not do sth

　　It is pretty cold. Youd better put on my coat.

　　Shed better not play with the dog.

　　had better have done sth表示與事實相反的結果，意為本來最好。

　　You had better have come earlier.