Learn Russian: К счастью, все вернулись домой.

While your current lesson focuses on opinion adverbs starting with по- (like по-моему), к счастью is another common way to express your attitude about a situation. It literally translates to "to happiness." Notice that it uses the preposition к followed by the dative case (счастью), which is structurally similar to how the по- adverbs also trigger the dative case!

Why is the verb вернулись in the plural form?

The pronoun все (everyone/all) is grammatically plural in Russian. Even though in English we treat "everyone" as a singular subject (saying "everyone is" or "everyone comes"), Russian always requires все to take a plural verb like вернулись.

I sometimes mix up дома and домой. Why do we use домой here?

You use дома when there is no motion, meaning "at home" (describing a location). You use домой when there is motion towards the home, meaning "homeward" (describing a direction). Because вернулись (returned) is an action of motion directed at a destination, домой is required here.

Why does the verb вернулись have the -сь ending?