Breakdown of Твой брат уже взрослый? Да, он взрослый.
брат
brother
он
he / it (for a masculine noun) / it (for a neuter noun)
да
yes
твой
your [informal] (твой, твоя, твоё)
Questions & Answers about Твой брат уже взрослый? Да, он взрослый.
Why is он взрослый translated as 'he is an adult' without a word for 'is'?
In Russian, the present tense of the verb 'to be' is almost always left out. You literally say 'he adult' (он взрослый) and 'your brother already adult' (твой брат уже взрослый).
Is взрослый an adjective or a noun here?
This lesson is about expressing age with the Dative case, like ему. Why do we use он here instead?
You only use the Dative case (like ему, meaning 'to him') when expressing a specific numerical age, such as ему двадцать лет (he is 20 years old). When you use a descriptive word for someone's stage of life, like взрослый (adult), you just use the basic Nominative case subject (он).
Why is it твой брат instead of твоя брат?
Possessive words like 'your' have to match the gender of the noun they describe. Since брат (brother) is a masculine noun, you must use the masculine form твой. If you were asking about a sister, it would be the feminine твоя сестра.