Breakdown of Посторонним нельзя находиться в здании.
здание
building
в
in / to (direction)
нельзя
one must not
находиться
to be located
Questions & Answers about Посторонним нельзя находиться в здании.
Why is "unauthorized people" in the dative case (посторонним)?
Sentences expressing rules or conditions using impersonal words like нельзя (forbidden), можно (allowed), or надо (necessary) require the person affected by the rule to be in the dative case.
Why does посторонним have an adjective ending if it acts as a noun here?
Посторонний (outsider/unauthorized person) is an adjective that functions as a noun. Even though it represents a person, it still takes standard adjective endings, which is why the dative plural is посторонним.
Why is the imperfective verb находиться used here?
When нельзя is followed by an imperfective verb, it means the action is "forbidden" or "not allowed," as in this sentence. If you used a perfective verb instead, the meaning of нельзя would change to "physically impossible."
Why does здании end in -ии instead of the usual prepositional -е?
The dictionary form is здание (building). Neuter nouns that end in -ие (along with feminine nouns ending in -ия) are exceptions to the standard prepositional case rules. They take an -и ending, resulting in в здании.