Learn Hardcore Korean: geuga choegeune undongeul sijakhan iyuga mwoya? - What’s the reason he recently started exercising?

The phrase 그가 최근에 운동을 시작한 is a relative clause modifying 이유. Inside relative clauses, the subject is normally marked with 이/가, so 그가 is the default.
If you use 그는, you’re topicalizing him at the sentence level: 그는 최근에 운동을 시작한 이유가 뭐야? means “As for him, what’s the reason…?” It’s acceptable but slightly different in structure/nuance from embedding him as the subject of the modifier.

Can I drop 그가?

Yes. Korean often omits pronouns when clear from context. 최근에 운동을 시작한 이유가 뭐야? is very natural.
In real conversation, speakers also prefer a name or title over 그. Use 그 only if the referent is already clear and you really need a pronoun.

Why ask with … 이유가 뭐야? instead of just using 왜?

Both are fine. 왜 …? is the direct adverb “why,” e.g., 왜 최근에 운동을 시작했어?
… 이유가 뭐야? literally asks “what is the reason,” which can feel a bit more explanatory or careful in tone. It invites an explicit reason rather than a simple cause.

What’s the difference among 최근에, 최근, and 요즘?

최근에 = “recently” (adverb). Great with a one-time change like 시작하다 (to start).
최근 = a noun/attributive meaning “recent; the recent…,” used as in 최근 운동 or with a particle (최근에).
요즘 = “these days,” implying an ongoing period or habit. Better with habitual actions (요즘 운동해) than a punctual “start.”

Why is it 시작한 and not something like 시작했는?

To modify a noun with a past action, Korean uses the past adnominal ending -ㄴ/은: 시작하다 → 시작한.
Forms like 시작했는 are ungrammatical. If you want a “remote/used-to” nuance, you’d use 시작했던 (see below on tense nuances).

Is 운동을 시작한 the only natural phrasing? What about 운동하기 시작한?

Both are natural:
- 운동을 시작하다 treats 운동 as a noun object.
- 운동하기 시작하다 uses the nominalizer -기 (“to start doing exercise”).
Nuance difference is minimal; choose what flows better in your sentence.

Why 이유가 and not 이유는?

This is an equational structure: A가 B야 (“A is B”). Here 이유가 is the grammatical subject.
이유는 topicalizes: “As for the reason, what is it?” It can add contrast or imply other aspects were discussed. 가 is the neutral/default; 는 adds topic/contrast.

Is 뭐야 casual? What are polite versions?

Yes. -야 is casual speech (banmal).
Polite options:
- … 뭐예요? (colloquial polite)
- … 이유가 무엇이에요? (polite, slightly more formal)
- … 이유가 무엇입니까? (formal/polite)

What’s the difference between 뭐 and 무엇? Why 뭐예요 and not 뭐이에요?

Can I change the word order, like 최근에 그가 운동을 시작한 이유가 뭐야??

Yes. Placing the time adverb first is common: 최근에 그가… is fine.
Splitting the object and verb (e.g., 그가 운동을 최근에 시작한 이유) is possible but can sound less smooth. Keep 최근에 near 시작한 for clarity.

Can I drop the object particle 을 in 운동을?

In casual speech, object markers are often dropped when meaning is clear: 운동 시작한.
Keep 을 in careful writing or whenever there might be ambiguity.

What changes if I use 시작하는 / 시작할 / 시작했던 instead of 시작한?

시작한: He already started; you’re asking the reason for that completed starting.
시작하는: Reason for (someone) starting in general/nowadays; used for habitual/general statements (e.g., “reasons people start exercising”).
시작할: Reason to start (before starting) or the reason he will start.
시작했던: Reason for having started back then (remote-past or backgrounded, often implying it’s no longer the case).

Can I use 무슨 instead of 뭐, as in 무슨 이유야??

무슨 modifies a noun and can mean “what/which kind of.” 무슨 이유야? is grammatical and asks “what kind of reason is it?”
In this sentence pattern, 이유가 뭐야? (equational “the reason is what?”) is more idiomatic. If you want to use 무슨, a common rephrasing is 무슨 이유로 (그가) 최근에 운동을 시작했어? (By what reason/for what reason did he start recently?).