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. Youd better put on my coat.
Shed 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.
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