Breakdown of Мой знакомый работает в банке.
мой
my / mine
работать
to work
банк
bank
знакомый
the acquaintance (m, as noun)
Questions & Answers about Мой знакомый работает в банке.
Why does знакомый end like an adjective if it is a noun meaning 'an acquaintance'?
Знакомый is a word that looks and changes its endings exactly like an adjective, but it functions as a noun. Russian does this with a few words, like столовая (dining room). Because it acts as a masculine noun here, the pronoun 'my' takes the masculine form мой.
Could I use друг instead of знакомый here?
You could, but it changes the meaning. In Russian, друг is reserved for close friends. For people you know casually, like coworkers, neighbors, or friends-of-friends, you use знакомый for a man or знакомая for a woman.
Should I use the reflexive pronoun свой instead of мой here?
No, because мой знакомый is the subject of the sentence (the person doing the working). You only use свой when the object of the sentence belongs to the subject. For example, if you said 'He loves his bank,' it would be Он любит свой банк.
Why is it в банке rather than на банке?
A bank is considered a physical, enclosed building and an institution, so it takes the preposition в (in/at) followed by the Prepositional case (банк becomes банке). The preposition на is usually for open spaces, surfaces, or events.