could anybody tell me how to differentiate them .when they are used as an noun.

for examples

1.She could see nothing in the dark.

2.The room was in complete darkness.

i hope to learn to separate their use easily.

i hope a can get many examples
AnonymousCould anybody tell me how to differentiate them when they are used as an noun?
"in the dark" is the typical usage of dark as a noun. "afraid of the dark" is another. It tends to be less truly noun-like than darkness because it usually means "dark time(s)" or "dark place(s)", so dark is usually really an adjective promoted to the status of noun.

It seems to me that dark always occurs unmodified; only darkness can be modified. Hence, "in the dark", but "in complete darkness". Further, as mentioned above, dark does not seem to occur very often in the subject or object positions of a sentence, but only after a preposition. dark in subject or object position seems to occur in the titles of films, plays, novels, and songs more than in ordinary conversation, perhaps because it sounds more poetic there.

Others may find good counterexamples.

I had a response typed out... and then realized it was so tricky. One of things that I was going to say was that "darkess" took modifiers about how dark it is (so that "ominious dark" doesn't really apply to the level of darkness), but then I got to "It was nearly dark" and gave up.

It's a hard one!
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This is a tricky question: in definitional terms the words mean pretty much the same thing, and the differences are of nuance (or custom) only. I don't think you'll find an easy rule that will always tell you which one is best to use. Some random thoughts...

The first thing that comes to my mind is that "darkness" has a richer and more atmospheric or dramatic feel. For example: "The old ruin loomed eerily out of the darkness", but "Carrots help you to see well in the dark". "Darkness" also seems more likely to be used figuratively, of a mood, say: "A darkness fell over him".

"Darkness" seems more likely to be used as the subject of a sentence: "Darkness covered the land"; "The darkness was impenetrable".

There are a number of idioms and stock phrases that dictate one word or the other, possibly for no very logical reason. For example: "complete darkness"; "in the dark" (unaware of something); "pitch dark"; "shot/leap in the dark"; and so on.
 CalifJim's reply was promoted to an answer.
CalifJimIt seems to me that dark always occurs unmodified; only darkness can be modified.

Some Googling throws up plenty of exceptions to this:

"He crouched in the moonless dark"

"Sleepless in the cold dark, I..."

"In the timeless dark of his captivity..."

"The ominous dark crept in still more..."

and so on.

But although these are all good English, to me they all have a poetic feel. I can't think of many good "everyday" examples apart from "pitch dark" (and even this, when a noun, has a tendency to be replaced by "pitch darkness"). "In the half-dark" is an expression I might possibly actually use in conversation.
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I can always count on you, Mr. Wordy, to keep me honest! Thanks for your input!

 BarbaraPA's reply was promoted to an answer.

I think that it can also be used to describe deep emotional turmoil as well.

"What could you possibly know of darkness like mine?" It can be used to describe guilt, loneliness, depression, hopelessness, a great loss.

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