Question 1

Why do we use под вечер here instead of к вечеру to mean towards evening?

Accepted Answer

While both can translate to towards evening, к вечеру (with the Dative case) often implies reaching a deadline or a result that accumulates by evening. Под вечер (using под + Accusative case) simply means approaching evening or late in the afternoon, acting purely as an approximation of time.

Question 2

Why is the verb увеличилась reflexive (ending in -сь)?

Accepted Answer

In Russian, if something changes state on its own, like a duration increasing, you use the reflexive form увеличиться. If you used the non-reflexive увеличила, it would mean the subject increased something else (a transitive action), which would require a direct object.

Question 3

What tells us that длительность is a feminine noun?

Accepted Answer

Nouns ending in the suffix -ость are almost always feminine abstract nouns. They are very common in Russian for expressing concepts and qualities, like длительность (duration). Because it is feminine, the past tense verb увеличилась takes the feminine -ась ending.

Question 4

Why is цикла in the Genitive case?

Accepted Answer

It answers the question duration of what? When linking two nouns where one belongs to or describes the other (the duration of the cycle), Russian uses the Genitive case for the second noun.

Под вечер длительность цикла увеличилась.

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