Questions & Answers about Мой друг остановился у нас на ночь.
Why do we use на ночь to mean "for the night"?
This is the focus of our current lesson! When you want to express the intended duration of an action's result (like how long someone plans to stay, or how long you borrow a book for), you use на + the Accusative case. Here, the friend arrived with the intention of staying for the duration of the night.
If на requires the Accusative case here, why does ночь not change its ending?
Ночь actually is in the Accusative case here! Because it is a feminine noun ending in a soft sign (-ь), it belongs to the 3rd declension group. For these nouns, the Accusative form is identical to the Nominative form.
Doesn't остановился mean "stopped"? Why does it translate to "stayed"?
Yes, the literal meaning of the perfective verb остановиться is "to stop." However, in Russian, this is the standard verb used for "staying" somewhere temporarily as a guest, such as at a hotel (в гостинице) or at a friend's house. You can think of it as "he stopped by our place to sleep."
Why is "with us" translated as у нас instead of с нами?
If you used с нами (with us / accompanied by us), it would mean he physically did the action of stopping or staying together with you as a group. У нас (literally "by us" or "at us") is the standard Russian idiom to say "at our place" or "at our house."