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Don't judge a book by its cover.
Literal
Book [object-を] cover [by-で] judge-[don't-な].
A direct calque of the English proverb 'don't judge a book by its cover.' The sentiment translates neatly; this is mostly a Western import into Japanese and not a traditional Japanese saying. 表紙 ('cover, front page'), で as means/instrument, 判断する ('to judge'), and the prohibitive particle ~な attached to the dictionary form. Note: this ~な is distinct from sentence-final な — it specifically marks prohibition when attached to a verb's dictionary form.