To say that one can't do something or that something is forbidden we use the word “нельзя” in Russian.
However, the grammar around it makes the sentences a little tricky. Here's why:
1) The aspect of the following verb will affect the meaning of the phrase.
To say that something isn’t allowed, use the infinitive form of the imperfective verb:
Здесь нельзя переходить дорогу. (It's not allowed to cross the road here.)
В ресторанах и кафе Киева нельзя курить. (It's not allowed to smoke in cafes and restaurants in Kyiv.)
When we mean that something can’t be done, simply because it is impossible, we use the infinitive of the perfective verb after “нельзя”: Кино нельзя посмотреть, дома нет электричества. (We can’t see a film, there is no electricity in the house.)Дверь нельзя открыть, замок сломан. (You can’t open the door, the lock is broken.)
2) Sentences with “нельзя” have no subject. The person (animal, etc.) who isn't allowed something is used in the Dative case, or isn't mentioned at all, if it's a general rule.
Ему нельзя есть мороженое. (He isn't allowed to eat ice cream.)
В музее нельзя фотографировать. (Taking pictures isn't allowed in the museum.)
3) And last, but not least. We don't say "не можно" in Russian!Нельзя говорить "не можно"!
