I’d better do something
It means, that if I don’t do something, there might be a problem or danger.
- You’d better be careful next time.
- I’d better go now, or I’ll be late.
I’d better not
- You don’t look very well. You’d better not.go at school today.
- You’d better not eat to much or you’ll get fat.
Even though had is normally past, in the meaning had better it is present or future.
Had better and should
- I don’t feel well. I’d better go home.
In this sentence we talk about a specific situation, not things in general.
- You’re always tired. You should go to bed earlier.
In this sentence we give an advice.
- It’s a great song. You should listen to it.
There is no danger or problem if you don’t follow the advice. It’s just a recommendation.
- I’d better be quiet or she will find me.
There might be a problem or a danger if she finds you (= if you don’t follow the advice).
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