Learn Russian: Кто-то прячется за диваном.

The preposition за means "behind". When it describes a static location (answering the question "where?"), it must be followed by a noun in the instrumental case. Since диван is a masculine noun ending in a consonant, it takes the -ом ending to become диваном.

Why does the verb прячется have a reflexive ending?

The base verb прятать means "to hide [an object]". Because the person in this sentence is hiding themselves, we need the reflexive form прятаться. For "he/she/it", the ending is -тся. They aren't hiding a toy; they are hiding themselves!

Why do we use кто-то here instead of кто-нибудь for "someone"?

Кто-то is used when we know a person definitely exists, but we just don't know their identity. The speaker knows there is absolutely a person behind the sofa! We would use кто-нибудь (which often translates to "anyone") if we weren't sure someone was there at all, like in the question: "Is there anyone behind the sofa?"

How is the end of прячется pronounced?

The letter combination -тся (and -ться) at the end of Russian verbs is pronounced like -ца (tsa). So, the whole word sounds like "прячеца".