eoje ilgdeon chaege chaekgalpireul kkoja dueosseoyo.

This is one of the biggest grammar points in the sentence.

• 읽은 책 = the book that I read / have read
  • This sounds more like the reading is viewed as a completed event.
• 읽던 책 = the book I was reading / had been reading
  • This often suggests:
    • the action was ongoing in the past,
    • it may have been unfinished,
    • or the speaker is recalling something from experience.

So 어제 읽던 책 most naturally means the book I was reading yesterday, not simply the book I read yesterday.

In this sentence, that nuance fits well because if you put a bookmark in it, you probably had not finished the book yet.

-던 is a retrospective modifier. It looks back on an action or state from the speaker’s memory.

With verbs, it commonly gives one or more of these meanings:

• ongoing in the past
  • 읽던 책 = the book I was reading
• habitual/repeated in the past
  • 자주 가던 카페 = the café I used to go to often
• recalled from experience
  • something the speaker remembers directly

In this sentence, 읽던 strongly suggests the book was in the middle of being read, not finished.

Yes, but the nuance changes.

• 읽던 책 = the book I was reading
• 읽었던 책 = the book I had read / the book I used to read / the book I read before

읽었던 usually sounds more clearly completed or more distant as a memory.

So:

• 어제 읽던 책 = the book I was reading yesterday
• 어제 읽었던 책 = the book I had read yesterday / the book I read yesterday

For a sentence about putting in a bookmark, 읽던 책 is more natural because a bookmark usually implies you plan to continue reading later.

Because 책갈피를 꽂다 works like to stick/insert a bookmark into a book.

Here the roles are:

• 책갈피를 = the thing being inserted → direct object
• 책에 = the place it goes into → location/destination marker

So:

• 책갈피를 꽂다 = insert a bookmark
• 책에 책갈피를 꽂다 = insert a bookmark into the book

Using 책을 would make book the direct object, which does not match the structure of 꽂다 here.

Both are possible, but they are not exactly the same.

• 꽂다 = to stick, insert, slot into something
• 넣다 = to put in

For bookmarks, 꽂다 sounds very natural because a bookmark is being slipped into place between pages.

So:

• 책에 책갈피를 꽂다 = natural, common way to say put a bookmark in a book
• 책 속에 책갈피를 넣다 = understandable, but less idiomatic for this specific action

English often uses a broad verb like put, but Korean often chooses a more specific action verb.

책갈피 means bookmark.

It is made from:

• 책 = book
• 갈피 = a fold, section, or division between pages/layers

So the word originally has the sense of something marking a place in a book.

It is the standard everyday word for bookmark.

This is another key point.

• 꽂았어요 = I put/inserted it
• 꽂아 두었어요 = I put/inserted it and left it there / kept it that way

The grammar -아/어 두다 means:

• do something in advance
• or do something and leave it in that state

So 책갈피를 꽂아 두었어요 means more than just the action of inserting the bookmark. It implies the bookmark was placed there so the place would be marked later.

That is exactly what bookmarks are for, so 꽂아 두었어요 sounds very natural.

Yes. 뒀어요 is the contracted form of 두었어요.

So these are equivalent:

• 꽂아 두었어요
• 꽂아 뒀어요

The contracted form is very common in speech.

Nuance:

• 두었어요 can sound a little more careful or written out
• 뒀어요 sounds more conversational

The ending -었어요 in 두었어요 makes the sentence:

• past tense
• polite informal style, often called 해요체

So the overall tone is polite and natural for everyday conversation.

Base form:

• 꽂아 두다

Past polite:

• 꽂아 두었어요

This is a very common speech level in Korean.

The subject is omitted, which is very normal in Korean.

From the verb ending and context, the most natural interpretation is:

• I put a bookmark in the book I was reading yesterday.

But depending on context, it could also mean:

• he/she/they put a bookmark...

Korean often leaves out subjects when they are already understood from the situation.

In this sentence, 어제 most naturally modifies 읽던.

So:

• 어제 읽던 책 = the book I was reading yesterday

It does not most naturally mean:

• yesterday, I put a bookmark in the book

That meaning would usually need clearer placement or context.

Because 어제 is directly before 읽던 책, learners should usually read it as belonging to the noun phrase:

• [어제 읽던 책]에

Yes, it would work, and it is a bit more explicit.

Compare:

• 어제 읽던 책 = the book I was reading yesterday
• 어제 읽고 있던 책 = the book I was in the middle of reading yesterday

읽고 있던 emphasizes the progressive aspect more directly.
But in many everyday situations, 읽던 책 already gives the right meaning and sounds more natural and compact.

Korean puts modifiers before the noun.

So:

• 어제 = yesterday
• 읽던 = was reading
• 책 = book

Together:

• 어제 읽던 책 = the book I was reading yesterday

This is a normal Korean pattern:

• 제가 어제 산 가방 = the bag I bought yesterday
• 친구가 만들던 음식 = the food my friend was making

English uses relative clauses after the noun; Korean usually uses verb modifiers before the noun.

Yes, that implication is quite strong.

Two parts support that idea:

• 읽던 책 suggests the book was not finished
• 책갈피를 꽂아 두었어요 suggests the speaker marked the place for later

So even if the meaning is not stated directly, the sentence strongly implies:

• the speaker stopped reading,
• marked the place,
• and may continue reading later.

Even if the basic meaning is already known, the sentence carries a very natural extra nuance:

• I put a bookmark in the book I was reading yesterday and left it there so I could come back to it.

So the Korean is not just describing a simple past action. It also conveys:

• past ongoing reading,
• an unfinished point in the book,
• and a resulting prepared state.