Learn Hardcore Korean: beoseuga eonje oneunji moreugesseoyo. - I don’t know when the bus is coming.

It marks an embedded/indirect question (unknown information) inside a bigger sentence. Here, 언제 오는지 means “when (it) comes,” treated like a noun clause. So 버스가 언제 오는지 모르겠어요 = “I don’t know when the bus is coming.” Common verbs that take this pattern: 알다, 모르다, 궁금하다, 물어보다.

Why is it 오는지 and not 올지?

-는지 with action verbs expresses unknown factual/scheduled/habitual timing in the present frame: 언제 오는지 “when it comes (is coming).”
-(으)ㄹ지 is more about future/possibility/guess: 언제 올지 “when it will come (if/ whether it’ll come, and when).”
Both can be heard; use -는지 for schedule/current timing, -(으)ㄹ지 for a yet-to-happen, uncertain future.

Why is the subject marker 가 used on 버스 instead of 는?

가 marks grammatical subject inside the embedded clause (버스가 (subject) 오는지). Using 는 would topicalize it: 버스는 언제 오는지 모르겠어요 shifts the focus to the bus as the topic (as opposed to, say, the subway). Both are possible; 가 is neutral inside such clauses.

Can I change the word order?

Yes. Natural variants:

버스가 언제 오는지 모르겠어요.
언제 버스가 오는지 모르겠어요. (slight emphasis on “when”)
If context makes “bus” obvious, you can drop it: 언제 오는지 모르겠어요.
The meaning stays the same; only emphasis changes slightly.

What’s the difference between 몰라요 and 모르겠어요?

몰라요 = “I don’t know.” Plain and direct.
모르겠어요 uses -겠- (supposition/stance), often sounding softer or less certain: “I’m not sure / I don’t think I know.”
Register variants: 모르겠어 (casual), 모르겠습니다 (formal). Adding 잘 softens: 잘 모르겠어요 “I’m not really sure.”

How do I say the positive version, “I know when the bus is coming”?

버스가 언제 오는지 알아요.
With more certainty/formality: 압니다 in very formal speech.

How do I talk about past or future?

Past unknown: 버스가 언제 왔는지 모르겠어요. “I don’t know when the bus came.”
Future/possibility unknown: 버스가 언제 올지 모르겠어요. “I don’t know when the bus will come.”

Is this -지 the same as the “since” -은 지 (e.g., 온 지 2시간)?

No. They’re different grammars:

-(으)ㄴ/는/ㄹ지 marks an embedded question/uncertainty (as in 오는지).
-은/ㄴ 지
- time expression means “since” (e.g., 버스가 온 지 2시간 됐어요 “It’s been 2 hours since the bus came”).

Can I say 언제인지 instead of 언제 오는지?

Not by itself here. 언제인지 means “when it is (what time it is)” and needs a clear referent, e.g., 버스가 오는 시간이 언제인지 모르겠어요 (“I don’t know what time the bus is coming”). If you keep 오다, the natural form is 언제 오는지.

Why isn’t it 와는지 (from 와요)?

Inside embedded clauses, you don’t use the polite -요 form. You use the plain attributive/conjunctive form: 오- + 는 + 지 → 오는지, not 와는지. The stem is 오-, while 와요 is a surface polite conjugation used only at sentence endings.

Can I drop 버스가?

Yes, if context makes it clear: 언제 오는지 모르겠어요. In Korean, known or obvious subjects are often omitted.

What other question words can I use with -는지?

어디에 가는지 몰라요. “I don’t know where (someone) is going.”
왜 늦는지 알아요? “Do you know why (they) are late?”
누가 오는지 몰라요. “I don’t know who is coming.”
얼마나 걸리는지 알아요? “Do you know how long it takes?”
몇 시에 시작하는지 모르겠어요. “I don’t know what time it starts.”

What’s the literal breakdown of the original sentence?

버스 bus + 가 (subject marker)
언제 when
오- come (verb stem) + -는지 (embedded question marker)
모르겠어요 I’m not sure / I don’t know (polite)
Altogether: “As for the bus, [the matter of] when it comes — I don’t know.”

Is -는가 possible instead of -는지?

Yes, but it’s formal/literary: 버스가 언제 오는가 모르겠다. In everyday speech, -는지 is natural.

How would I ask the direct question instead of embedding it?

버스가 언제 와요? or 언제 버스가 와요? = “When is the bus coming?”
The original sentence embeds that question under 모르겠어요 to say you don’t know the answer.