grammar - What is the difference between べからず and べからざる?

In my JLPT practise book, I have the following question:

A：噂にまどわされる＿＿＿＿＿、とは言うものの・・・。

B：つい、信じちゃうよね。

1. べく　2.べき　3.べからざる　4. べからず

I knew the answer was between 3 and 4, but I couldn't make a final decision on why I would choose one over the other. According to the book, the answer is 4.

According to the book's explanation, at least as I understand it, べからざる means something can't be allowed, and べからず means something must not happen.

However, both have a certain amount of logical sense in the question above. Assuming I understand it correctly (which I probably don't), it's either saying "we can not allow ourselves to be deceived by rumours", or "we can't be deceived by rumours", depending on whether we use べからざる or べからず.

What is the difference that I'm not seeing?

Answer

SUMMARY

Options 3 & 4 both mean "must not" because they are both variations on べきではない　tested at JPLT N1 (in fact べからざる　is a variation on べからず (see on)). The trick is to understand which best fits the context of the sentence when we apply "usual" JLPT level N1 definitions but even if we do not fully understand the context we can still get the question right if we know the grammatical rules (接続の仕方).

TECHNICAL DEFINITIONS

べからず is used for injunctions (ie prohibitions such as "Do not..") and is an old written form not generally used but sometimes written on bulletin board/notices. As a result it is quite likely to appear in a quotation. eg:

(公演で）「芝生に入るべからず」｜([Notice] at the park) "Keep off the grass"

べからざる　is the form of べからず　that modifies nouns ie VるべからざるN. eg:

彼は我がチームには欠くべからざる選手である。｜Mr A is (an) indispensable (player) on the team.

(And, just for the sake of completeness, べき　is the form of べく　that modifies nouns ie VるべきN.]

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APPLICATION (COMPREHENSIVE ANSWER)

If we apply these principles to the question:

A：「噂にまどわされる＿＿＿＿＿、とは言うものの・・・。」

B：「つい、信じちゃうよね。」

1.べく　2.べき　3.べからざる　4. べからず

Step 1: Read and (try to*) understand the context of the passage to eliminate the obvious incorrect answers:

A: "Although it says _ be distracted by the rumours,...."

B: "I just took them as face value, didn't I."

The gap is most likely to be filled by a prohibition (ie an injunction such as "Do not...") rather than a command ("Do...."). This eliminates options 1 & 2.

Step 2: Look at next option: 3. べからず　is used for injunctions and this is a quotation so without looking at 4 we know this is a good candidate for the correct answer.

Step 3: Option 3 is confirmed as correct by looking at the remaining option: 4 べからざる is just a variation on 3. べからず which is used to modify nouns (VるべからざるN). Although it literally means the same as 3 grammatically it does not fit as well / at JLPT level N1.

*Note: I am sure somebody can improve on my translation but it is good enough for the question!

References: The references I used to write this answer were 日本語表現文型辞典　＆　総まとめ　文法N1 but once you've done enough questions you can intuitively "guess" the right answer just using step one (that's what I did here and last July).