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I came across this sentence while checking a student's English journal:
"It was unforgettable eight days."

What sounds more natural to me would be:
"It was an unforgettable eight days."

However, "eight days" is obviously plural while "an" can only be singular. Is there a certain grammar rule that allows these to be different in number yet still grammatical?

marked as duplicate by Araucaria, Community Jun 2 '18 at 13:20

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