I'd like to suggest accepting "quickly" for быстро, as quickly and быстро are adverbs, and "fast" and быстрый are adjectives.
Well, technically 'fast' here is an adverb that has the same form as the adjective (most dictionaries list adverbial fast alongist with the adjective), but I agree that 'quickly' should be accepted too.
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Russian word order is flexible and allows some variation. If you want to make a generic statement, Дети быстро растут is more common (where "grow fast" is a single statement you want to make about kids). If you stress that children grow at a very high rate, you may as well move the adverb to the end of the sentence. Since both interpretations make sense, we accept both.
Apparently the word быстро is the origin of the French word "bistrot" meaning a small restaurant /café.
Wiki extract: The word derived from the Russian bystro (быстро), "quickly". It entered the French language during the Battle of Paris (1814). Russian officers or cossacks who wanted to be served quickly would shout "bystro."
Both forms are correct, but they mean different things:
- Расти́ ‘to grow’ (3rd pl. расту́т) means ‘to become bigger/more mature’,
- Расти́ть ‘to grow’ (3rd pl. растя́т) mean ‘to let someone/something become bigger/more mature’.
- Де́ти расту́т ‘The children are growing’
- Де́ти растя́т цветы́ ‘The children are growing flowers’.
Расти́ть/растя́т describes an action directed to some outer object, расти́/расту́т describes an action directed at oneself.
I believe it's not... :?
I'm not sure, different languages mark stressed syllables differently, but I can't tell much about the Russian stress, sorry. But this sentence definitely has the correct stress.
Because 'children' is plural, so it need to be used with the plural verb form (grow). 'Grows' is a singular verb form, you'd use it with singular nouns (the child grows fast).