Breakdown of К этому документу нужно добавить около ста страниц.
этот
this [f: эта, n: это]
документ
document
страница
page
нужно
it is necessary
Questions & Answers about К этому документу нужно добавить около ста страниц.
Why is the word for 'a hundred' ста here instead of сто?
The preposition около (about/around) always requires the Genitive case. The number 100 is very simple to decline: it is сто in the Nominative and Accusative, but changes to ста in all other cases (Genitive, Dative, Instrumental, and Prepositional). So here, it becomes ста.
Since ста is in the Genitive case, is страниц in the Genitive plural just because numbers 5 and up take the Genitive plural?
Not exactly! When a number is in an oblique case (any case besides Nominative or Accusative), the rule changes. The noun no longer just takes the Genitive; instead, it has to match the number's case in the plural. Because the preposition около forces the Genitive case here, both the number (ста) and the noun (страниц, Genitive plural of страница) happen to be Genitive. If we were using the Dative case, it would be ста страницам.
Why do we use the Dative case for этому документу?
The preposition к (to/towards) always takes the Dative case. When we add something 'to' something else in Russian, we use the phrase добавить к, which changes этот документ to the Dative этому документу.
Why doesn't the Russian sentence explicitly say 'we' (like мы должны) when the English translation says 'We need to'?