Is "hidden" redundant here?

"Secret" doesn't mean that it is not there. Is there not pills in a bottle if we don't hear them shack around in the bottle? "Behind" would just mean blocked by like the bottle. For another question if you stick your hand into a cardboard box; is it not there?

The "reason why" and the "reason that", is a very easy on to answer. The "reason why" we do something is because it is knowledge, something we are taught. The "reason that" we do something is because it is our wisdom. Wisdom is something we learn, but not like knowledge, because knowledge is what we know about something. Wisdom is what we have had in our lives. We have never seen or anything about a dinosaur, but we have knowledge of them.

Thank you.

In your opinions, they are all necessary and indispensable?

Things can be hidden in plain sight. You could put a secret in a box behind a Christmas tree, but if its hidden, you must search for it.
There is always a story behind events, but if it is a secret it is often hidden, and you'll have to dig for the information.

Yes. The reason that . . .

Notice the fine, detailed explanation Dave has given, Ori. When a fella like Dave has been so wrong, so many times on language issues, it should give you great pause. When Dave offers you his typical quick answer with no explanation whatsoever, it should also give you that same pause.

Dave is simply repeating someone else's nonsense. You must have read this nonsense yourself in some style manual at some time or other or you wouldn't have even known to ask.

Interesting directions for grammatical studies:

1) The Usefulness of the Useless
2) The Efficientness of the Inefficient
3) The Succinctness of the Redundant

OmSigDavid in a nutshell. "His" ideas on language, stolen from others, provide an easy route to knocking down these idiotic notions.