不得【bùdé】

不得 forbids an action and goes before the verb. It belongs to laws, contracts, regulations and signs; the matching obligation word is 应当. In speech, people say 不能, 不准 or 别.

The forbidden thing is often a two-syllable verb, and the sentence often opens with the condition under which it is forbidden: 未经同意, 在…期间, 任何人.

不得 or 不得不

One more 不 reverses it completely. 不得不 means "have no choice but to".

• × 这里不得不吸烟。 → 这里不得吸烟。

A rule, not an event

不得 states a standing rule, so it takes no 了 and doesn't report what someone failed to do.

• × 他昨天不得进去。 → 他昨天没能进去。
• × 不得停车了。 → 不得停车。