You had better go to see a doctor right now为什么是had不是have?

You had better go to see a doctor right now为什么是had不是have?
You had better go to see a doctor right now
为什么是had不是have?

You had better go to see a doctor right now为什么是had不是have?
这是固定的,在提建议的时候用You had...常写为You'd better ...

固定用法，had better do sth 最好做某事,没什么原因
比如You'd better close the window