Placed before a verb, marks the action as forbidden by a rule or standing prohibition — may not, must not.
框 · Frame
[subj / rule] 不得 [verb phrase]
觸 · Trigger
A regulation forbids an action; you state the action as off-limits, not merely undone.
序 · The move
1Name the action a rule blocks.Is this a standing prohibition, not just something that did not happen?
2Set 不得 directly before that verb phrase.Does a regulation or authority stand behind the ban?
3Keep the register formal — notices, contracts, rules.Would 不能 / 不可以 read too conversational for this line?
例 · Examples
1這家公司規定stipulate / rule員工employee每天工作時間不得may not / must not超過exceed八小時。
This company's rules state that an employee's daily working hours may not exceed eight.
界 · Boundary
不能
不得 is a rule forbidding it (you are not permitted); 不能 is inability or general can't (you are not able). 不得 carries an authority behind the ban; 不能 needs none.
不要
不得 is a standing prohibition in formal text; 不要 is a spoken directive to one listener (don't). 不得 names what is not allowed; 不要 tells someone to stop.
不得很累 ✗ → 不能很累 / 很累 ✓ (不得 forbids an action, not a state)
你不得來嗎？ ✗ → 你不能來嗎？ ✓ (不得 is not used in casual questions)
我今天不得去 ✗ → 我今天不能去 ✓ (personal inability takes 不能, not the rule-ban 不得)
English 'may not' covers both permission and possibility; 不得 is only the permission/prohibition sense. Reach for 不能 / 可能不 when you mean possibility.