Learn Russian: Нам нужно идти, потому что здесь нельзя шуметь. - We need to go, because making noise here is not allowed.

Нельзя is an impersonal word meaning "it is not allowed" or "forbidden." In Russian, you simply pair it with an infinitive verb, like шуметь (to make noise), to express that an action is prohibited. You don't need a dummy subject like "it" or "you" before it.

Why do we use the dative pronoun нам instead of мы (we)?

Words like нужно (necessary) and нельзя (forbidden) are impersonal, meaning they don't take a standard nominative subject. Instead, the person experiencing the need or restriction is put in the Dative case. So "we need" becomes нам нужно (literally "to us it is necessary").

Why is the verb шуметь in the imperfective aspect here?

When you use нельзя to say that an action is forbidden or not allowed, you almost always use an imperfective verb like шуметь. If you used a perfective verb with нельзя, it would change the meaning from "you are not allowed to do it" to "it is physically impossible to do it."

Why use идти instead of another verb of motion like ходить?

Идти is a unidirectional verb, meaning motion in a single direction. Since the speakers need to leave this specific place right now, they are making a single, one-way trip, making идти the correct choice. Ходить would imply walking around aimlessly or making round trips.