てはならないFormal prohibition
てはならない states that an action must not be done, presenting the ban as a formal rule, public principle, or serious standard rather than a personal request.
A rule, not merely a preference
てはならない makes the prohibition sound authoritative and grounded in a rule or general standard. It is therefore natural when an institution controls conduct, or when a speaker presents something as a serious principle that everyone should observe.
Institutional rule
社内規程により、職員は顧客情報を私的に利用してはならない。
Under company regulations, employees must not use customer information for personal purposes.
AnalyzeSerious general principle
事実を確認せずに、他人を責めてはならない。
One must not blame others without checking the facts.
AnalyzeThis sounds like a general standard of conduct, not simply the speaker's personal request.
Formal spoken use
我々は過去の過ちを忘れてはならない。
We must not forget the mistakes of the past.
AnalyzeThis is natural in a speech or other serious public statement.
Choosing the right kind of prohibition
Use てはならない when the ban is presented as formal or authoritative. てはいけない is the neutral everyday choice and is much more natural in ordinary interaction. べきではない instead judges an action to be wrong or improper; it does not necessarily state an actual rule.
More examples
図書館内では、携帯電話で通話してはならない。
Phone calls must not be made inside the library.
Analyze作業中は安全装置を外してはならない。
The safety device must not be removed while work is in progress.
Analyze運転者は勤務中に酒を飲んではならない。
Drivers must not drink alcohol while on duty.
Analyze十分な証拠がない段階で、結論を急いではならない。
One must not rush to a conclusion before there is sufficient evidence.
Analyze患者の同意なしに、個人情報を公開してはならない。
Personal information must not be disclosed without the patient's consent.
Analyzeどのような状況でも、人の尊厳を傷つけてはならない。
A person's dignity must not be harmed under any circumstances.
Analyze