As far as I know, "the + adjective" form is used to refer to the people or things belonging to the adjective group as in "The poor have many concerns", but can we say just "the poor" to refer to only one poor person? as in "The poor is there"?
I don't believe so. When poor is used as a noun, it is a plural noun. org/dictionary/english/money to the poor.
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I don't believe so. When poor is used as a noun, it is a plural noun. See Cambridge Dictionary: https://dictionary.cambridge.org/dictionary/english/poor
fire1but can we say just "the poor" to refer to only one poor person?
Impossible.
CJ