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The modern literary language is usually considered to date from the time of Alexander Pushkin (Алекса́ндр Пу́шкин) in the first third of the 19th century. Pushkin revolutionized Russian literature by rejecting archaic grammar and vocabulary (so-called "высо́кий стиль" — "high style") in favor of grammar and vocabulary found in the spoken language of the time. Even modern readers of younger age may only experience slight difficulties understanding some words in Pushkin's texts, since relatively few words used by Pushkin have become archaic or changed meaning. In fact, many expressions used by Russian writers of the early 19th century, in particular Pushkin, Mikhail Lermontov (Михаи́л Ле́рмонтов), Nikolai Gogol (Никола́й Го́голь), Aleksander Griboyedov (Алекса́ндр Грибое́дов), became proverbs or sayings which can be frequently found even in modern Russian colloquial speech.

What happened to some expressions in Pushkin's writing?
became proverbs or sayings which can be frequently found even in modern Russian colloquial speech