A avea is the dictionary form meaning to have. Because the subject is tu (you), the verb changes its shape to match. The informal 'you' form of this verb is ai.
Where is the word for 'Do' in this question?
Unlike English, Romanian doesn't use extra words like do or does to ask a question. You simply take the statement form (Tu ai un frate) and add a question mark, raising the pitch of your voice at the end when speaking.
Is it possible to drop the word Tu and just say Ai un frate?
Yes, absolutely! Because the verbai can only mean that you have something, the word tu is optional. Romanian speakers usually leave it out unless they want to add emphasis to who exactly they are asking.
Why do we use un here instead of o?
Un is the Romanian word for a or an when used with masculine or neuternouns. Because frate (brother) is masculine, it takes un. If you were asking about a sister (soră), which is feminine, you would use o instead.
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