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Plenty isn't an adjective, but a noun meaning a large number or amount. The adjective is plentiful: Oranges are cheap now because they are plentiful
So is an adverb, and must qualify an adjective or another adverb. Such is an adjective and must qualify a noun
Truth isn't an adjective but a noun. The adjective is true, and we use it with no article between it and the verb to be.
Fool is a noun, and requires the article when we use it with the verb to be. Foolish is an adjective, and can't be used with the article after the verb to be.
A fool or a foolish person doesn't mean an insane person, but one who acts thoughtlessly. We tend to use silly or stupid instead of foolish in modern usage.