べからず

You know how we can attach べき to a VERB to mean "should VERB," yeah?

If not, then put your review pants on and go read this: JLPT N3: べきだ.

You also know that the verb ending ～ず means ～ない, yeah? That is, it's a negative verb ending. JLPT N2 and N1 tests love the ～ず verb ending.

↑ Smash all that together, and you get ↓

JLPT N1: べからず (must not)

Placing べからず after a VERB changes the meaning to "must not VERB" or "cannot VERB" or "VERB not allowed."

This is an outdated, written form. I doubt you'll ever hear people saying it, and you certainly don't need to be using it in your own Japanese. You may, however, see it printed on a sign every now and then.

Like this:

Instead of 入るべからず, it would be more common to write something like 入ってはいけません (はいってはいけません // Do not enter.) or perhaps even just 入らないでください (はいらないでください // Please do not enter.).

If you're old school, though, perhaps you would opt to write 入るべからず.

Wait a second.

Didn't we already study this?

Mmm... not quite. But we studied something very similar: JLPT N1: べからざる.

📚 Grammar Stuff 📚

V る ＋ べからず
must not VERB; cannot VERB

Just like we saw with べきだ, when the VERB before べからず is する, we say すべからず or するべからず.

Writing this lesson, I wondered: Why does べからず mean "must not" or "cannot?"

Like you, I too ask questions that teachers hate answering.

So I looked up べからず in a dictionary:

べからず：〔推量の助動詞「べし」の未然形「べから」に打ち消しの助動詞「ず」の付いたもの〕
助動詞「べし」の打ち消しの言い方。現代語でも文語的表現として用いられることがある。
べからず comes from adding the negative ending ず to the imperfective form べから of the conjectural helping verb べし. It is a negative form of the helping verb べし. It is sometimes used in modern written language.

To summarize, the dictionary was telling me:

Some examples:

Enough!

Oh, and maybe I should have mentioned this before, but you might also want to look at this lesson on べく, which has quite a different meaning than べきだ 、べからざる、and べからず：JLPT N1: べく.

Get confused べからず, yo.