Learn Russian: Неужели в автобусе кто-то наступил тебе на ногу? - Did someone really step on your foot in the bus?

This is the dative of inalienable possession. When an action negatively affects a body part, Russian usually treats the person as the recipient of the detriment. So instead of saying someone stepped on 'your foot', Russian says they stepped 'to you' (тебе in the dative case) on the foot. This emphasizes how the action personally impacted you.

Why is it кто-то here instead of кто-нибудь for 'someone' in a question?

Even though this is a question, the action already happened in the past and the speaker likely sees the physical result (like you limping). They are asking about a specific, real person who actually did the stepping, even if their identity is unknown. Кто-нибудь is used for purely hypothetical people, typically in requests or general questions about the future.

Could I use Разве instead of Неужели to mean 'really'?

You could, but Неужели conveys a much stronger sense of shock or disbelief. While Разве is often used to clarify a fact that contradicts your expectations, Неужели emphasizes pure surprise, as if saying 'Is it actually possible that someone stepped on your foot?'

Why does на ногу take the accusative case instead of the prepositional?