情態動詞：had better表示

had better 相當於一個助動詞，它只有一種形式，它後面要跟動詞原形。
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表示與事實相反的結果，意為"本來最好"。
you had better have come earlier.