Нельзя выбегать vs нельзя выбежать
I was very pleased to see how easy you made the the quiz on verbs of motions with prefixes - congratulations!
However, there was one particularly difficult question, and almost half of you made a mistake:
So, what is the actual difference between нельзя выбегать на дорогу and нельзя выбежать на дорогу?
Нельзя may mean both "not allowed, may not" and "physically impossible, can not".
If нельзя goes with an imperfective verb, it means "not allowed".
Нельзя выбегать (imp) на дорогу means "don't run onto the road!". This is what moms usually say to their children, because roads are dangerous for small kids.
If нельзя goes with a perfective verb, it means "physically impossible", "you can't"
Нельзя выбежать (perf) на дорогу means that the road is inaccessible, you can't run onto it - for example, because it's fenced off from pedestrians.
Another example with нельзя and perfective or imperfective verbs:
Нельзя нарушать законы! - You should not break the laws!
Нарушать is the imperfective, and obviously this sentence is referring to human laws, such as those spelled out in the criminal code and so on.
Нельзя нарушить законы физики! - You cannot break the laws of physics. But you can try to do it, it is not a crime!
This is how the verbal aspect changes the meaning of нельзя.