Is the below sentence grammatically correct?
Know not what you know.
closed as off topic by MετάEd, Hellion, Kristina Lopez, aedia λ, cornbread ninja 麵包忍者 Jun 13 '13 at 16:12
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(line from an old hymn)
Here, the archaic negative imperative is used - but the construction is just that, archaic. Except for effect / old times' sake, the regular negation ('do not tarry') (or more idiomatically, 'be quick about it') would always be chosen nowadays.
If a declarative sentence was intended, the construction is again archaic:
He knows not what to do. (archaic)
He doesn't know what to do. (modern idiomatic)