This pattern expresses a strong prohibition or something that should not be done. It is archaic and extremely formal, found mostly in written Japanese such as rules, notices, contracts, or literature. In speech, it sounds unnatural or humorous, so avoid using it in conversation.

べからず is used at the end of a sentence or clause. For example, '入るべからず' means 'Do not enter.' べからざる is the attributive form, meaning it comes before a noun, like '欠くべからざる存在' meaning 'an indispensable presence' (literally, 'a presence that must not be lacking'). Remember that for する verbs, you use す, so '勉強すべからず' is correct, not '勉強するべからず'.