Learn Russian: Директор занят, поэтому мы не входим в кабинет. - The director is busy, therefore we are not entering the office.

Входим is the imperfective verb for "to enter." We use it here because the action is happening right now in the present tense ("we are not entering"). The perfective verb войти cannot be used in the present tense; if we used its conjugated form войдём, it would mean "we will enter" in the future.

Why do we see the word в twice in входим в кабинет?

This is a very common pattern in Russian! When using a prefixed verb of motion like входить (to enter, literally "to in-walk"), you usually repeat the prefix as a standalone preposition right after the verb. So, you always "in-walk" в (into) your destination.

Why is кабинет in the accusative case here instead of the prepositional case?

Even though the preposition в can mean "in" or "inside" (which takes the prepositional case), here it shows direction or destination ("into"). When you are moving towards a destination, you must use the accusative case. Since кабинет is a masculine inanimate noun, its accusative form looks exactly like its dictionary form.

Why does the sentence use занят instead of the full adjective занятый?