How to Express Prohibition and Impossibility

Did you know that if you use the word “нельзя” (“it is not allowed”) with a verb in perfective mood, it means physical impossibility to do something, but, if you use the word “нельзя” with a verb in imperfective aspect, it means prohibition.

For example,

Нельзя открыть дверь. Она закрыта на ключ.
It is impossible to open the door. It is locked.

Нельзя открывать дверь. В комнате спит ребёнок.
You are not allowed to open the door. The child is sleeping in the room.