Prohibition: нельзя + Imperfective
The word нельзя means "must not" or "cannot," depending on the aspect of the verb that follows it.
Paired with an imperfective verb, нельзя expresses prohibition. An action is forbidden by a rule, boundary, or the law (закон). Because the imperfective focuses on the process, this structure means: "Do not engage in this action at all."
| the law | закон |
| It is forbidden to smoke here because it is forbidden to break the law. | Здесь нельзя курить, потому что нельзя нарушать закон. |
| It is forbidden to take photos in this museum. | В этом музее нельзя фотографировать. |
| It is forbidden to enter this territory without a passport. | На эту территорию нельзя входить без паспорта. |
Impossibility: нельзя + Perfective
Paired with a perfective verb, the meaning shifts to impossibility. Because the perfective focuses on a successful result, this structure means: "It cannot be successfully done."
This describes physical limits—like when an object is too heavy to lift (поднять) or move (сдвинуть), or too strong to destroy (разрушить).
| The box is too heavy, it cannot be lifted. | Ящик слишком тяжёлый, его нельзя поднять. |
| This huge stone cannot be moved. | Этот огромный камень нельзя сдвинуть. |
| This old wall cannot be destroyed, it is made of iron. | Эту старую стену нельзя разрушить, она из железа. |
This also applies to abstract impossibilities, such as when there is no evidence to prove (доказать) a fact, or when it is impossible to predict (предсказать) the future.
| This fact cannot be proved because there are no documents. | Этот факт нельзя доказать, потому что нет документов. |
| The future cannot be predicted, one can only hope. | Будущее нельзя предсказать, можно только надеяться. |
Being Strict
Whether enforcing a regulation or pointing out a physical limit, use the adverb строго (strictly) to describe how rigidly a rule is applied or checked.
| Things are strict at our place: one must not smoke here. | У нас строго: здесь нельзя курить. |
| It is impossible to hide the secret, the director strictly checks the report. | Скрыть тайну нельзя, директор строго проверяет отчёт. |