Breakdown of Через месяц мы вернём треть суммы.
мы
we
месяц
month
вернуть
to return
через
in (after a period of time)
Questions & Answers about Через месяц мы вернём треть суммы.
Why does сумма change to суммы after треть?
The word треть (a third) is a noun. In Russian, when you have a fraction noun like половина (half), треть (a third), or четверть (a quarter), the thing being divided must be in the Genitive case. So, the Nominative сумма (sum) becomes the Genitive суммы (of the sum).
Why didn't треть change its ending if it's the direct object of the sentence?
It actually is in the Accusative case here because it is the direct object of вернём (we will return). However, треть is a feminine noun ending in a soft sign (a 3rd declension noun, like вещь or дверь). For these inanimate nouns, the Accusative form looks exactly the same as the Nominative form.
Why is the preposition через used here to mean "in" a month?
While через often means "across" or "through" in physical space, it has a very specific rule for time. When talking about how much time will pass before a future event happens, через followed by the Accusative case means "in [amount of time]" or "[time] from now". Thus, через месяц translates perfectly to "in a month".
How do we know вернём means "will return" (future) and not "return" (present)?