за своим столом

at one's desk

→ за
phrase Rare #44,872
at one's desk Стол can mean either “table” or “desk”; своим means “one’s own” and refers to the subject of the sentence.
RU: Она читает за своим столом.
EN: She is reading at her desk.

Том работал за своим столом, когда я его видел последний раз.

Tom was working at his desk the last time I saw him.

Сестра сидит за своим столом.

The sister is sitting at her desk.

Мэри работает за своим столом.

Mary is working at her desk.