Learn Russian: На улице нельзя мусорить.

Нельзя is an impersonal word meaning 'it is forbidden' or 'one must not'. When you want to say that an action is not allowed, you just pair нельзя directly with the infinitive of the verb, like мусорить. You do not need a subject like 'you' or 'people'.

Why is the verb мусорить in the imperfective form?

When нельзя means 'it is forbidden', it is almost always followed by an imperfective verb. This is because it expresses a general rule: you should not be doing this action at all, ever.

Why does the sentence start with На улице instead of Нельзя?

In Russian, the background information or setting usually goes at the beginning of the sentence, while the most important new information goes at the end. By starting with На улице, the speaker sets the scene first, placing the strong emphasis on the rule against littering.

What case is улице in, and why?

It is in the Prepositional case. The preposition на followed by the Prepositional case is used to show location, answering the question 'where?'. Since the rule applies to being located on the street, we use на улице.