べからずDo not, Must not, Should not, Cannot (べからざる)

べからず and てはいけない are similar in that they both express prohibition, or the idea that you 'must not' or 'cannot' do something. The difference is in formality and how they connect to other words. べからず is a highly formal and archaic expression that nowadays is found mostly in regulations, proverbs, and so forth. It follows directly after the plain form of a verb, e.g. 入るべからず ('Do not enter.'). In contrast, てはいけない is far more common in the modern language, and connects to the て-form of verbs, e.g. 入ってはいけない ('You must not enter.')

Both indicate prohibition. べからず is old-fashioned and literary, often seen on signs or notices, as in 立入るべからず (Do not enter). てはならない is the standard modern form, still formal but usable in both speech and writing, as in 入ってはならない (You must not enter).