Learn Icelandic: Er einhver inni?

In this sentence, einhver is the subject of the verb er (is), so it must be in the nominative case. It is also in the masculine singular form, which Icelandic uses as the default grammatical gender when referring to an unknown person.

The translation says someone, but wouldn't we normally say anyone in English?

Yes, and einhver means both! Unlike English, which usually switches from someone to anyone in questions, Icelandic simply uses einhver for both statements and questions.

Why do we use inni instead of inn for inside?

Icelandic uses different adverbs depending on whether you are talking about a stationary location or an active movement. Because we are asking if a person is already located inside, we use the stationary word inni. If you were asking if someone was going inside, you would use the movement word inn.