doseogwane ganeun gime chaekdo bannaphalgeyo.

-(으)ㄴ/는 김에 means something like while you’re at it, since you’re already doing that, or on the occasion of doing that.

So 도서관에 가는 김에 means: since I’m going to the library anyway / while I’m on the way to the library

It suggests that going to the library is the main action, and returning the book is an extra thing done at the same time.

Because 김에 normally attaches to a modifier form:

• verb stem + 는 김에 for action verbs
• adjective/past forms can behave differently, but with a verb like 가다, you use 가는 김에

So:

• 가다 → 가는 김에
• 먹다 → 먹는 김에
• 만나다 → 만나는 김에

Even though the whole sentence refers to a future action, Korean still uses 가는 김에 here, not 갈 김에.

김에 is a grammar pattern made from the noun 김 plus the particle 에.

In modern Korean, learners usually just memorize it as the expression -(으)ㄴ/는 김에. It has the sense of using an opportunity or occasion to do something additional.

Pattern:

• A-(으)ㄴ/는 김에 B

Meaning:

• Since A is happening anyway, do B too
• While doing A, also do B

In this sentence:

• A = going to the library
• B = returning the book

도서관 means library, and 에 marks the destination of movement.

So:

• 도서관에 가다 = to go to the library

With movement verbs like 가다, 오다, 다니다, 에 often marks the place you go to.

Examples:

• 학교에 가요 = I go to school
• 집에 와요 = I come home

So 도서관에 가는 김에 literally starts as on the occasion of going to the library.

도 means also, too, or even, depending on context.

So 책도 반납할게요 means:

• I’ll return the book too / as well

It implies there may be another action already happening, and returning the book is added to it.

Here, the main plan is going to the library, and 책도 marks returning the book as an additional action.

Compare:

• 책을 반납할게요 = I’ll return the book
• 책도 반납할게요 = I’ll return the book too / as well

In Korean, 도 usually replaces particles like 을/를 rather than coming after them.

So:

• natural: 책도 반납할게요
• unnatural: 책을도 반납할게요

This is a common pattern:

• 밥을 먹어요 → 밥도 먹어요
• 영화를 봐요 → 영화도 봐요

So in this sentence, 책도 means the book too.

반납하다 means to return something officially, especially something borrowed from an institution or organization.

It is commonly used for:

• library books
• rental items
• uniforms
• ID cards
• equipment

So it fits very naturally with 도서관.

돌려주다 is more general and often means to give back to a person.

Compare:

• 도서관 책을 반납하다 = return a library book
• 친구한테 책을 돌려주다 = give a book back to a friend

So 반납하다 is the better choice here.

반납할게요 uses -ㄹ게요, which often expresses:

• the speaker’s intention
• a decision made on the spot
• a promise or assurance to the listener

So 반납할게요 feels like:

• I’ll return it
• I’ll go ahead and return it

It often has a slightly more personal or responsive feeling than -ㄹ 거예요.

Compare:

• 반납할게요 = I’ll return it
  → can sound like a promise or something the speaker is volunteering to do
• 반납할 거예요 = I’m going to return it
  → more like a neutral future plan

In this sentence, 반납할게요 sounds natural because the speaker is telling someone what they will do.

Korean often omits the subject when it is clear from context.

So even though the sentence does not explicitly say I, the ending -ㄹ게요 strongly suggests the speaker is talking about their own action:

• 도서관에 가는 김에 책도 반납할게요
• Since I’m going to the library anyway, I’ll return the book too

If needed, you could add a subject:

• 제가 도서관에 가는 김에 책도 반납할게요

But in normal conversation, leaving it out is very common and natural.

Yes. It is in a polite conversational style.

The key part is 반납할게요, which ends in -요, making it polite but not formal-stiff.

So the tone is suitable for:

• everyday conversation
• speaking to someone you should be polite to
• casual polite situations

It is less formal than something like:

• 반납하겠습니다

and more conversational/natural in many daily contexts.

No. It can be used with many kinds of actions, not just movement.

It always gives the idea of doing something extra because the first action is already happening.

Examples:

• 마트에 가는 김에 우유 좀 사 올게요.
  Since I’m going to the mart anyway, I’ll buy some milk too.

• 청소하는 김에 창문도 닦았어요.
  While I was cleaning, I cleaned the windows too.

• 전화하는 김에 그 사람한테도 연락해 봐.
  Since you’re calling anyway, try contacting that person too.

So in your sentence, going to the library is just the first action that creates the opportunity for the second one.