Breakdown of На эту территорию нельзя входить без паспорта.
этот
this [f: эта, n: это]
на
on, at / by (transport) / to (a place) / to (in answer to)
паспорт
passport
нельзя
one must not
Questions & Answers about На эту территорию нельзя входить без паспорта.
Why do we use the imperfective verb входить here instead of the perfective войти?
This is the key rule for нельзя! When you pair нельзя with an imperfective verb like входить, it means the action is forbidden or against the rules. If you used the perfective verb войти, it would mean the action is physically impossible to complete (for example, if a concrete wall was blocking the only entrance).
Why does the sentence use the accusative case на эту территорию instead of the prepositional на этой территории?
Because the verb входить (to enter) involves motion toward a destination. When the preposition на indicates the direction or destination of a movement (answering the question 'where to?'), it requires the accusative case. If you were describing something happening while already located inside the territory (answering 'where at?'), you would use the prepositional case.
Could we use the verb заходить instead of входить here?
Yes, they are very similar, but входить is a better fit for this sentence. Входить sounds more strict and official, which matches the context of border control or security checks. Заходить can feel a bit more casual, like 'dropping by' or wandering into an area.
Just a quick refresher: why is паспорт in the genitive case (паспорта)?