Learn Hardcore Korean: doseogwan naeeseoneun hyudaeponeul kkeoya haeyo. - Inside the library, you have to turn off your phone.

It’s 에서 (at/in, where an action happens) + 는 (topic/contrast marker). Adding 는 makes “inside the library” the topic and often adds a contrastive feel: “As for inside the library (at least), you must turn off phones.” Without 는, 도서관 내에서 is a plain location.

Why is it 에서 and not 에?

에서: location where an action happens (turning the phone off is an action).
에: static location or destination. So “turn off (your phone) in the library” needs 에서.

What exactly is 내, and can I use 안 instead?

내 (內) is a Sino‑Korean bound noun meaning “inside,” a bit formal/written.
안 is the native word for “inside,” more everyday. All are fine:
도서관 내에서 (formal-ish)
도서관 안에서 (neutral/conversational)
Even 도서관 내부에서 (more formal/technical “interior”).

Is there any difference between 도서관 내에서는 and just 도서관에서는?

Both can work. 도서관에서는 means “at/in the library.” 내에서는 makes “inside” explicit, which can matter if there are adjoining spaces (courtyard, lobby area) where the rule may differ.

How does 꺼야 해요 express “have to/must”?

It’s the grammar V + 아/어야 하다 = “must/have to V.”

끄다 (to turn off) → stem 끄-
- 어야 하다 → 끄어야 하다 → contraction 꺼야 하다 → polite 꺼야 해요.

Why is it 꺼야 and not 끄어야?

끄다 is an ㅡ‑irregular verb. With endings starting in 어/아, the ㅡ drops and you get vowel contraction:

끄다 → 꺼요 / 껐어요 / 꺼야 해요 Similar patterns: 쓰다 → 써요, 크다 → 커요.

Can I say 꺼야 돼요 instead of 꺼야 해요?

Yes: V-아/어야 돼요 is also “must/have to.” Many speakers use 해야 해요 and 해야 돼요 interchangeably. Some feel 돼요 can sound a touch more like an external requirement, but the difference is small. Note the spelling: 돼요 (not “되요”).

Would a different ending be more natural on a sign?

Yes, public notices often use requests/imperatives:

휴대폰을 끄세요. (polite request)
휴대폰을 끄십시오. / 꺼 주시기 바랍니다. (formal)
Very formal: 휴대전화 사용을 금합니다. (“Use is prohibited.”)

Do I need the object particle 을 after 휴대폰?

Grammatically yes (휴대폰을). In casual speech it’s often dropped (휴대폰 꺼야 해요), but in writing and notices it’s better to keep it.

Is 휴대폰 the best word here? What about 핸드폰 or 휴대전화?

휴대폰: common and neutral.
핸드폰: very common in conversation (colloquial).
휴대전화: more formal/official. Any would be understood; signage often prefers 휴대전화 or 휴대폰.

Who is the subject? It isn’t stated.

Korean often drops generic subjects. Here it means “you/people in general” must turn off phones. You could add 여러분은 or 이용객들은, but it’s usually omitted.

How do I pronounce the whole sentence naturally?

Revised Romanization: Doseogwan nae-eseoneun hyudaepon-eul kkeoya haeyo
Tips:
- 휴대폰을 → [휴대포늘] (linking makes it sound like “po-neul”)
- 꺼야 해요 → [꺼야 해요]
- 도서관 내에서는 → [도서관 내에선] (the final 는 is often light)

Can I emphasize the obligation more strongly?

Yes:

Add an adverb: 반드시/꼭 휴대폰을 꺼야 해요.
Use -어야만 하다: 휴대폰을 꺼야만 해요.
Prohibition style: 휴대전화 사용을 금합니다.

Is the spacing in 도서관 내에서는 correct?

Yes. With the bound noun 내, standard spacing is 도서관 내 (space), then particles attach: 도서관 내에서는. You’ll see 도서관내 in the wild, but formal writing prefers the spaced form.