Learn Hardcore Korean: doseogwaneseoneun joyonghi iyagihae jwo.

-에서 marks the place where an action happens.
Adding the topic marker -은/는 makes -에서는, which topicalizes or contrasts that place: “As for in the library…”
With -에서는, the sentence can sound like a general principle or a contrast with other places.

Why is it 도서관에서 and not 도서관에?

-에 is for static location/existence or destination.
-에서 is for where an action occurs. Since talking is an action, use -에서.

Does -에서는 make it sound like a rule?

Yes. Topic-marking the location often implies a norm: “In the library (as a rule), talk quietly.” Without -는, it can feel more like a one-time, here-and-now request.

What does -아/어 주다 (as in 이야기해 줘) add?

It’s a benefactive: “do X for me/for someone.” In requests, it softens a command into “please do X.” So 이야기해 = “talk,” while 이야기해 줘 ≈ “please talk (for me/for us).” The beneficiary is usually understood and omitted.

How polite is 줘? What are the safer polite versions?

Casual: …해 줘
Polite (informal polite): …해 줘요
Standard polite request: …해 주세요
More deferential: …해 주시겠어요? / …해 주시겠습니까?
Very formal/signage: 정숙해 주십시오 (for “Please be quiet.”)

Is there a difference between …해 줘요 and …해 주세요?

Both are polite, but …해 주세요 is the default “please do …” request and feels a bit more courteous/formulaic. …해 줘요 is fine but slightly less formal in tone.

Why use 이야기하다 instead of 말하다? Which sounds more natural here?

이야기하다 = to talk/converse.
말하다 = to speak/say/tell. With 조용히, both work. If you mean “keep your voice down while talking,” 조용히 이야기해 (줘) is fine. If you simply want lower volume, 조용히 말해 (줘) also fits. For “Be quiet,” Koreans very often say 조용히 해 (줘) or use 정숙 in formal notices.

Why 조용히 and not 조용하게?

Both exist, but 조용히 is the standard adverb for “quietly,” especially with speech verbs. 조용하게 is grammatical and can sound more descriptive/stylistic; in this request, 조용히 is the natural choice.

Can I say 얘기 instead of 이야기?

Yes. 얘기 is a very common contraction of 이야기. Same meaning. 도서관에서는 조용히 얘기해 줘 is perfectly natural in casual speech.

Should 해 줘 be written together or separately?

Both 해 줘 and 해줘 are widely used. Writing it separately treats 주다 as an auxiliary verb; many textbooks prefer the spaced form, but both are acceptable in practice.

Can 에서는 be shortened to 에선?

Yes. 에선 is a common spoken and informal written contraction: 도서관에선 조용히…

Who is the subject here? There’s no “you.”

Imperatives in Korean usually omit the subject; it’s understood as “you.” The beneficiary of 주다 is typically the speaker or the situation, so it reads as a polite request to the listener.

Where should 조용히 go in the sentence?

Place it right before the verb phrase by default: 조용히 이야기해 줘. You can front it for emphasis (조용히, …) but the given order is the most neutral and natural.

Could I add 좀 to sound softer?

Yes. 도서관에서는 조용히 좀 이야기해 줘 (or polite: …조용히 좀 이야기해 주세요) sounds more gently pleading. Note that 좀 can also imply slight impatience depending on tone, but it’s a very common softener.

Is the original sentence natural, or would Koreans say something else?

It’s natural to a friend/peer. In real life you’ll also hear:

도서관에서는 조용히 해 줘 (Please be quiet in the library.)
Polite: 도서관에서는 조용히 해 주세요.
Signage/formal: 도서관에서는 정숙해 주십시오.