The guideline is that some adjectives and adverbs are non-gradable: they cannot be less or more so, but just so or not so. Perfect is one example-- I am either perfect or imperfect ; supposedly, I cannot be a little bit perfect or very perfect . Your grammar book should contain a representative list of descriptive modifiers that are generally considered non-gradable: words such as perfect, unique, impossible, dead -- and also adjectives like Japanese, mechanical, metallic , etc.
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