eoje ilgdeon soseoreun chaekgalpireul kkoja dugo nawasseo.

읽던 is the noun-modifying form of 읽다 with -던, which points to an action that was ongoing, repeated, or unfinished in the past.

So 어제 읽던 소설 means something like:

• the novel I was reading yesterday
• the novel I had been in the middle of reading yesterday

The key nuance is that the reading was not simply treated as a completed event. It feels like the speaker is referring back to something they were engaged in.

This is a very common question.

• 읽은 소설 = the novel that I read
  This sounds more like the reading is completed.
• 읽던 소설 = the novel I was reading / had been reading
  This suggests an ongoing or unfinished action in the past.

In this sentence, the speaker put in a bookmark and left, so the idea of not being finished yet fits very naturally. That is why 읽던 is a better choice than 읽은 here.

Both refer to the past, but the nuance is different.

• 읽던 often emphasizes that the action was in progress, habitual, or unfinished at that past time.
• 읽었던 often sounds more clearly like a past experience or something completed in the past and now being recalled.

So:

• 읽던 소설 = the novel I was reading
• 읽었던 소설 = the novel I used to read / had read

In this sentence, 읽던 is more natural because the bookmark shows the speaker planned to continue reading later.

은 is the topic particle.

Here, 소설은 sets up the novel as the topic of the sentence. It is not necessarily the grammatical subject in the same way English works. Korean often uses a topic-comment structure.

So the sentence is roughly structured like:

• As for the novel I was reading yesterday, I put in a bookmark and left.

Using 은 can make the noun sound like something already known or being singled out for discussion.

-아/어 두다 means doing something and leaving it that way, often:

• in preparation for later
• for convenience
• with the resulting state maintained

So:

• 꽂다 = to stick/insert
• 꽂아 두다 = to insert it and leave it there

That is exactly what you do with a bookmark: you place it in the book so the position is preserved for later.

So 책갈피를 꽂아 두고 is not just put in a bookmark, but more specifically put in a bookmark and left it there for later use.

를 marks 책갈피 as the direct object of 꽂아 두고.

In other words, the thing being inserted is the bookmark.

• 책갈피를 꽂다 = to insert a bookmark

Even though English may focus on the book, Korean here focuses on the bookmark as the object being placed.

This depends on point of view.

• 나오다 = to come out / exit
• 가다 = to go

나왔어 means the speaker is describing the action as coming out from inside a place. For example, if they were inside a room, house, library, or classroom, leaving that place can be expressed with 나오다.

So the nuance is:

• not just I went
• but I came out / stepped out / left

In Korean, 나오다 is very natural when the important image is exiting an interior space.

The subject is not stated, but it is understood from context, usually as I.

Korean often omits subjects and objects when they are obvious.

So the full idea is something like:

• I put a bookmark in the novel I was reading yesterday and left.

The sentence sounds natural without 내가 because Korean does not repeat obvious information unless it needs emphasis or contrast.

The connector -고 links actions in sequence.

So:

• 책갈피를 꽂아 두고
• 나왔어

means:

1. I put in a bookmark and left it there
2. then I came out / left

This is a very common Korean way to show one action happening before another.

In this sentence, the first action prepares for the second one nicely: the speaker marks the page, then leaves.

In practice, learners usually understand 어제 as going with 읽던 소설:

• the novel I was reading yesterday

That is the most natural reading here.

Korean adverbs can sometimes have a wider scope depending on context, but in this sentence 어제 most naturally tells us when the reading was happening, not when the speaker left.

If the speaker wanted to make yesterday clearly refer to the leaving action, they would usually phrase the sentence differently or rely on context.

나왔어 is in the informal casual style, often called 반말.

The plain dictionary form is 나오다.
Past tense polite would be:

• 나왔어요

Past tense casual is:

• 나왔어

So this sentence sounds like something said to a friend, younger person, or someone with whom the speaker speaks casually.

Yes, it is natural.

꽂다 literally means to stick or insert something into something else, and it is commonly used for things like bookmarks, pens, flowers, and similar objects.

With 책갈피, it gives the image of placing the bookmark into the book at the correct page.

Because the sentence also uses 꽂아 두다, it sounds especially natural: the speaker inserted the bookmark and left it there to keep the place.

It is a mix of both.

• 어제 읽던 소설은 gives background information about which novel we are talking about.
• 책갈피를 꽂아 두고 나왔어 gives the main event.

So the sentence first identifies the novel, then tells what the speaker did with it.

This is a very common Korean pattern:

• identify a noun with a modifier
• mark it as topic
• say what happened in relation to it