Breakdown of Кто-то смотрит на нас через щель в двери!
дверь
door
в
in / to (direction)
на
on, at / by (transport) / to (a place) / to (in answer to)
смотреть
to watch / to look at / make sure (in a warning)
Questions & Answers about Кто-то смотрит на нас через щель в двери!
Since через takes the accusative case, why hasn't щель changed its ending?
Через does take the accusative case here to mean "through." However, щель (crack or slit) is a feminine noun ending in a soft sign (-ь). For these types of nouns (like дверь or ночь), the accusative form is identical to the nominative (dictionary) form, so it stays щель.
Could we use кто-нибудь instead of кто-то for "someone" in this sentence?
No, you need кто-то here because it refers to a specific but unidentified person—there is definitely someone looking! You would use кто-нибудь ("anyone" or "somebody") when the person's existence is hypothetical, like in a question: "Is someone looking?"
Does на take the prepositional case in the phrase смотрит на нас?
No, на takes the accusative case here (на нас). While на is often used with the prepositional case to mean "located on," when it shows the direction of an action—like directing your gaze at someone—it requires the accusative case.
Why does the prepositional case of дверь end in -и (в двери) instead of -е?