Learn English – “As well you know”

When expressing frustration or exasperation, we often begin by saying something like, "If I had only known..."

• If I had known your plane was going to be two hours late, I wouldn't have rushed to the airport.
• If I had known you were going to get such bad grades on your report card, I wouldn't have let you go to the dance last weekend.
• Had I known the van was going to overheat, I would have driven the car instead.

Yet, in all these instances, life offers no crystal ball; there's no way of knowing what the future holds, or to be cognizant of something we are unaware of. This is the essence of "You don't know what you don't know," only it's being expressed in a comical way, much like Yogi Berra might have said.

(To those unfamiliar with the Yogi Berra reference, he was a professional baseball player who was reknowned for his wry way of saying things, such as, "It ain't over 'til it's over," and, "You can observe a lot by watching." That said, you couldn't know what you didn't know, but now you know.)

Generally speaking, it would not be a good way to express something in formal writing, unless perhaps you were deliberately injecting humor.

The verbs need and dare are sometimes called "semi-modal", in that they sometimes behave like modal verbs (eg can: forming their negative and interrogative without do, and not taking -s in the third person singular).

The degree to which they do so varies between speakers, and according to register (degree of formality).

The idioms you mention ("Need I say more" etc.) are widespread.

In Britain I/you/he need not and I/you/he dare not are common, but I believe they are less common in American English.

"Need you", as in Need you go so soon? is old fashioned in Britain, but is still heard; I don't know about America.