Breakdown of Мой брат был очень занят и засиделся за компьютером.
и
and
брат
brother
мой
my (мой, моя, моё) / mine
компьютер
computer
Questions & Answers about Мой брат был очень занят и засиделся за компьютером.
How does the за- ... -ся pattern change the meaning of сидеть (to sit) in this sentence?
The prefix за- combined with the postfix -ся expresses absorption in an action, often to the point of losing track of time or doing it to excess. Here, сидеть (to sit) becomes засидеться, meaning to sit too long, get carried away, or stay up late doing something.
Does засидеться always mean staying up late at night?
Not necessarily, though it is a very common translation. It fundamentally means you sat somewhere for too long or overstayed. For example, if you visit a friend and lose track of time during the day, you can say я засиделся (I stayed too long). The context of being busy at the computer here implies staying up late.
Why do we use за компьютером (instrumental case) to mean at the computer?
The preposition за followed by the instrumental case literally means behind, but it is the standard Russian way to express sitting at a workstation, desk, or table. Using a literal translation like на компьютере (prepositional case) would sound like he was physically sitting on top of the machine!
Why use the short form adjective занят instead of the long form занятый?