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In some senses, incomprehensible is marked as literary. Watch for register when choosing this word.
VERBS
be, seem | remain
find sth
Many students find the instructions for the exam completely incomprehensible.
ADV
completely, quite, totally, utterly | almost, nearly, virtually | largely | pretty
PREP
to
The instructions in the manual were utterly incomprehensible to most customers without technical training.