adjective
a word that describes or gives more information about a noun, in this case 'ununderstandable' describes something that cannot be understood
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The complex scientific theory was ununderstandable to most of the students.
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The instructions were so ununderstandable that we had to ask for help.
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The new math concept is totally ununderstandable to me.
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His emotions were so mixed up and ununderstandable that no one could figure him out.