Breakdown of Ошибка была в ста документах, а не в пятнадцати.
не
not
а
and (contrastive)
в
in / to (direction)
ошибка
mistake
быть
to be
документ
document
пятнадцать
fifteen
сто
one hundred
Questions & Answers about Ошибка была в ста документах, а не в пятнадцати.
Why is it ста instead of сто?
The number сто is very special because it has only two forms! In the Nominative and Accusative cases, it is сто. For all other cases (the oblique cases, like Genitive, Dative, Instrumental, and Prepositional), it changes to ста. Since it follows the preposition в to indicate location, it needs the Prepositional case here.
Wait, shouldn't it be документов (Genitive plural) after a number like 100?
That rule only applies when the number is in the Nominative or inanimate Accusative case (for example: У меня есть сто документов). In all other cases, the number and the noun match in case! Here, the preposition в forces the Prepositional case, so both the number (ста) and the noun (документах) are in the Prepositional plural.
How do you decline a number like пятнадцать in this sentence?
Numbers from 5 to 20 decline exactly like feminine nouns ending in a soft sign (the 3rd declension, like тетрадь or вещь). In the Prepositional case, these nouns take the ending -и. That gives us в пятнадцати.
Why does the sentence use а не instead of и не for 'and not'?
When you are directly contrasting two things or correcting a statement (meaning 'it was X, not Y'), Russian uses the conjunction а. While English says 'and not', using и не in Russian would sound unnatural here because а specifically sets up a direct contrast or replacement.
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