chinguga i chaegeun soseolchaegirago haesseoyo.

Because the sentence has two layers:

• 친구가 = the main subject of the whole sentence: the friend
• 이 책은 소설책이라고 = the content of what the friend said

So the structure is basically:

• 친구가 ... 했어요 = A friend said ...
• Inside what was said: 이 책은 소설책이다 = This book is a novel

In other words, 가 marks who did the saying, and 은 marks the topic inside the quoted statement.

Using 가 here puts focus on the friend as the one who said it.

• 친구가 ... 했어요 = My friend / a friend said ...
• 친구는 ... 했어요 would sound more like as for my friend, they said ..., possibly contrasting the friend with someone else

So 친구가 is the most neutral and natural choice if you are simply reporting who said it.

은/는 marks the topic, while 이/가 marks the subject.

Here, inside the quoted statement, the speaker is talking about this book, so 이 책은 means something like:

• As for this book, it is a novel

This is very natural in Korean when introducing or discussing something as the topic.

If you said 이 책이 소설책이라고, that would feel more like emphasizing this book as the subject, often in contrast with something else. It is not impossible, but 이 책은 is the more expected form in this sentence.

-이라고 했어요 is a reported-speech pattern used after a noun.

It means:

• said that it is/was ...
• said, “It’s ...”

So:

• 소설책이라고 했어요 = said that it is a novel / said, “It’s a novel.”

The dictionary form is:

• -이라고 하다 after nouns ending in a consonant
• -라고 하다 after nouns ending in a vowel

Since 소설책 ends in ㄱ, it becomes 소설책이라고.

Because 소설책 is a noun.

Korean uses different quotation forms depending on what comes before them:

• noun + (이)라고 하다
• descriptive verb / verb clause + 다고 하다

So:

• 소설책이라고 했어요 = correct, because 소설책 is a noun
• 재미있다고 했어요 = correct, because 재미있다 is an adjective/descriptive verb
• 읽었다고 했어요 = correct, because 읽었다 is a verb clause

A simple way to remember it:

• noun → (이)라고
• sentence with a verb/adjective → 다고

It comes from 하다, which means to do, but in this pattern 하다 also works like to say in quoted speech expressions.

Steps:

• 하다 → 해요 (present polite)
• 했다 / 했어요 (past polite)

So 했다고 했어요 means said in the past.

In this sentence:

• ...이라고 했어요 = said that ...

So the whole sentence is in polite past style.

They are very close, but not exactly identical.

• 소설 = novel / fiction
• 소설책 = literally novel-book, meaning a novel book or simply a novel

In everyday Korean, 소설 is often enough. 소설책 can sound a little more explicit because it includes 책 (book).

So both can work, but:

• 이 책은 소설이라고 했어요
• 이 책은 소설책이라고 했어요

Both are understandable. The second one just spells out more clearly that it is a book in the novel category.

The direct quotation would be something like:

• 친구가 “이 책은 소설책이에요”라고 했어요.

That means:

• My friend said, “This book is a novel.”

Your original sentence is already using a quotation pattern, but without quotation marks, which is very common in Korean.

So:

• 친구가 이 책은 소설책이라고 했어요 and
• 친구가 “이 책은 소설책이에요”라고 했어요

are closely related. The second just shows the quoted words more explicitly.

When Korean reports speech after a noun, it often attaches (이)라고 directly to the noun.

So instead of saying:

• 소설책이다고 ❌

Korean says:

• 소설책이라고 ✅

And for direct speech, you could have:

• 소설책이에요 inside quotation marks

But when converting to this reported form, the noun goes straight into (이)라고 하다.

That is just how this grammar pattern works.

Yes, but the nuance changes.

• 이 책은 소설책이라고 했어요 = As for this book, my friend said it’s a novel
• 이 책이 소설책이라고 했어요 = My friend said that this book is the novel / is a novel, with more focus on this book as the subject

The 은 version sounds more natural in many everyday contexts because it presents this book as the topic of discussion.

The 이 version can sound more specific or contrastive, depending on context.

The Korean word order is:

• 친구가 = friend + subject marker
• 이 책은 = this book + topic marker
• 소설책이라고 = that it is a novel
• 했어요 = said

So literally, it is closer to:

• Friend-subject this book-topic novel-that said

Natural English rearranges this as:

• My friend said that this book is a novel.

Korean often places the quoted or descriptive content before the final verb.

Yes. Korean often leaves out possessives like my, your, or his/her when the meaning is obvious from context.

So 친구가 can mean:

• a friend
• my friend
• the friend

depending on the situation.

In many textbook translations, it is often rendered as my friend said..., because that is the most natural interpretation in everyday conversation unless another friend has already been specified.

It is polite informal style, because it ends in -어요.

• 했어요 = polite
• not super formal like 했습니다
• not casual like 했어

So this is a very common everyday polite sentence.

Examples:

• 친구가 이 책은 소설책이라고 했어요. = polite everyday
• 친구가 이 책은 소설책이라고 했습니다. = more formal
• 친구가 이 책은 소설책이라고 했어. = casual