21.9 had better表示"最好"，相当于一个助动词，它只有一种形式，它后面要跟动词原形。例如：
It is pretty cold. You'd better put on my coat. 天相当冷。你最好穿上我的外套。
She'd better not play with the dog. 她最好不要玩耍那条狗。
had better have done sth表示与事实相反的结果，意为"本来最好"。例如：
You had better have come earlier. 你本该来得早一点。