Breakdown of Как только они вернулись, они рассказали сыновьям о верности.
они
they
о
about (о, об, обо)
сын
son
рассказать
to tell (perfective)
Questions & Answers about Как только они вернулись, они рассказали сыновьям о верности.
Why does 'sons' look like сыновьям here?
The verb рассказали (told) requires the dative case for the recipient of the telling. The word for 'son' (сын) has an irregular plural сыновья. In the dative plural, these irregular nouns that end in -ья take the ending -ьям, making it сыновьям.
Why is it о верности instead of о верность?
When talking about something, the preposition о requires the prepositional case. Верность (faithfulness) is a feminine noun ending in a soft sign (-ь). These 3rd-declension nouns take the ending -и in the prepositional case, giving us о верности.
Does как только always mean 'as soon as'?
Yes, when used together as a conjunction, как только means 'as soon as'. It shows that one action happens immediately after another, which is why it pairs perfectly with the completed perfective action вернулись (returned).
Why is there a -сь at the end of вернулись?
Вернуться (to return) is a reflexive verb. Since the subject они (they) is plural, the past tense verb gets the plural ending -ли. The reflexive particle -ся shrinks to -сь after a vowel, resulting in вернулись.