Learn Russian: Вы видите этот знак? Здесь нельзя мусорить. - Do you see this sign? Littering is not allowed here.

Нельзя means "it is not allowed" or "forbidden". It is an impersonal word, which means it doesn't need a subject like "I" or "you" to do the action. You simply pair it directly with an infinitive verb—in this case, мусорить (to litter)—to say that the action itself is prohibited.

Why is the imperfective verb мусорить used after нельзя?

When нельзя is used to mean "it is forbidden" or "you must not", Russian almost always uses the imperfective aspect. This emphasizes that the action is completely and generally prohibited at all times, not just on a single occasion.

Doesn't the verb видеть (to see) require the accusative case? Why does этот знак look like the dictionary form?

It is exactly in the accusative case! Because знак (sign) is a masculine inanimate noun (it is an object, not a person or an animal), its accusative form is identical to its nominative (dictionary) form. The demonstrative pronoun этот (this) also follows this "no change" rule for inanimate masculine objects.

The verb видеть ends in -еть. Why is the "you" form видите and not видете?

Видеть is one of a few common exception verbs. Even though it ends in -еть, which normally signals the first conjugation (-е- endings), it actually belongs to the second conjugation. That is why it takes the -ите ending for the вы form, just like verbs ending in -ить.