Had better

[Tdol]

BNC results:

1 I HAD BETTER 68
1 I BETTER 138

[riverkid]

BNC results:

1 I HAD BETTER 68 0.68
1 I BETTER 138 1.38

What do those numbers mean, Tdol? Did you check <I'd better>

If it's not too much trouble, could you run,

You had better

You'd better

You better

[Red5]

I voted for option 2: had better not.

[MikeNewYork]

I voted for option 2: had better not.

Me too.

[beastmaster]

had better is a modal verb it looks like past but the meaning is present it is astrong advice and after had better not this is the correct one

[oana1486]

this is the correct form.

[csharp]

The American Heritage&#174; Book of English Usage. 1996. Page 18
You had better not do that or You’d better not do that.

[Teia]

Does anyone really understand this structure? To be honest, it escapes me.

You had better not do it = You`d better not do it = You better [in spoken English]not do it.

This idiomatic expression might sound as a recommendation or as a warning:

You had better not do it.[or else you`ll get into trouble]
You had better not do it. [or else I`ll punish you]

[Teia]

No
You'd better not do it. (present/future)
You'd better not have done it. (past)

Is this second sentence commonly used in spoken English? I think that English speakers prefer shouldn`t have done although there`s a slight difference [ if any?] between them.

[Tdol]

Yes, it is used; it can convey more force than 'shouldn't'.