Learn Hardcore Russian: Мой начальник редко задерживается после работы.

Russian uses the reflexive ending -ся/-сь to show that the subject is affected by the action or that the verb is intransitive. Задерживаться means “to stay (late), to be delayed, to linger.” The non‑reflexive задерживать means “to delay/hold back someone or something.”

Он задерживается на работе. = He stays late at work.
Полиция задержала нарушителя. = The police detained the violator.
Начальник задерживает сотрудников. = The boss delays the employees.

Why is the present tense used? Isn’t it about a repeated/habitual action?

Yes. In Russian, the present tense of an imperfective verb is the normal way to talk about habits and repeated actions. With adverbs of frequency like редко, часто, обычно, you generally use the present imperfective: Он редко задерживается = He rarely stays (habitually).

Why not use the perfective future задержится?

Задержится (perfective) refers to a single, completed event, typically in the future: Он редко задержится = He will rarely end up staying (on specific future occasions). For general habits, Russian prefers the imperfective present: Он редко задерживается.

What case is работы in, and why?

Работы is genitive singular. The preposition always takes the genitive: , , .