Questions & Answers about Кто-то стучит, я выйду во двор.
Why does я выйду mean "I will go out" instead of "I am going out"?
выйду is the perfective verb выйти (to go out). Because perfective verbs focus on the completion of an action, their present tense form actually creates the future tense! If you wanted to say "I am going out" right now, you would use the imperfective verb выхожу.
Since the verb has the вы- (exit) prefix, why do we use the preposition во instead of из?
The prefix вы- means "out of", but the preposition you use after the verb depends on where the person is going. The speaker is exiting the house and going into the courtyard. For motion into a place, we use в (or во) plus the accusative case. If they were walking out of the courtyard, you would use из двора.
Why is it во двор instead of just в двор?
It is just to make pronunciation easier. When a word starts with certain double consonants (like дв), Russian often adds an о to the preposition. Saying в двор is a bit of a tongue-twister, so it becomes во двор.
Why do we use кто-то here instead of кто-нибудь?