Questions & Answers about Наш самолёт улетает в семь утра.
The lesson is about the prefix от- (departure). Why does this sentence use улетает instead of a verb with от-?
The prefix от- means moving a short distance away from a specific object or point. However, the prefix у- means leaving a place completely to go somewhere else. For scheduled departures where a vehicle leaves for a different destination, Russian uses у- (literally "flies away").
The plane leaves in the future, so why is улетает in the present tense (imperfective)?
Just like in English ("Our plane leaves..."), Russian uses the present tense of imperfective verbs of motion for scheduled, fixed future events, such as timetables for flights, trains, and buses.
Why do we use в for "at seven", and what case is семь?
To say "at" a specific hour on the clock, Russian uses в + the Accusative case. The word семь is a numeral, and in the Accusative case, it looks exactly the same as its basic Nominative form.
Why is it утра instead of a prepositional phrase for "in the morning"?
When you attach a part of the day to a specific hour to clarify AM or PM (like "7 a.m."), you use the Genitive case. Утра is the Genitive of утро (morning). You are literally saying "at seven of the morning".