Learn Hardcore Korean: i chaegeun neomu jaemiisseoyo.

은 is the topic marker. It tells the listener “as for this book…” and sets 이 책 apart as the thing we’re talking about. Whenever you see 은/는, expect the speaker to present information or an opinion about that topic.

What’s the difference between 은/는 and 이/가?

은/는 marks the topic or contrasts items (“as for …”).
이/가 marks the grammatical subject and often introduces new information.
Using 이 책은 places emphasis on your overall comment about the book, whereas 이 책이 would simply identify the book as the subject of some action or description.

Can I drop 이 and just say 책은 너무 재미있어요?

Yes. 책은 is perfectly fine. Adding 이 (“this”) simply specifies “this particular book.” Without 이, you’re saying “books, in general, are very fun” or “the book (previously mentioned) is very fun.”

What does 너무 mean here?

Colloquially, 너무 means “very” or “really.” Though it originally meant “too (much),” in everyday speech it’s used as an intensifier without a negative nuance: “very interesting.”

Can 너무 ever mean “too much” rather than “very”?

Yes, in more precise or formal contexts 너무 can still carry the “excessive” meaning (“too much”). If you want to stress “too” as in “excess,” you might also see 지나치게. But in casual conversation, when paired with positive adjectives, 너무 almost always means “very.”

What part of speech is 재미있어요, and how is it formed?

재미있어요 is a descriptive verb (often called an adjective in English explanations). Its dictionary form is 재미있다 (“to be fun/interesting”), literally “there is fun.” You conjugate it like any other 하다‐type descriptive verb.

Why does it end with -어요?

-어요 is the polite informal ending (존댓말). You could also say:
• 재미있습니다 (formal polite)
• 재미있어 (casual/informal)
Choose your ending based on whom you’re speaking to and the level of formality.

How would you say “This book is boring”?

Simply use the opposite descriptive verb 재미없다:
이 책은 재미없어요.
This means “This book is not interesting” or “This book is boring.”

Can I replace 너무 with another adverb like 정말 or 아주?

Absolutely. You can say:
• 이 책은 정말 재미있어요. (really)
• 이 책은 아주 재미있어요. (very)
Each carries a slightly different nuance but all intensify “interesting.”

Could I say 이 책이 너무 재미있어요 instead, and what’s the nuance?

Yes, you can. 이 책이 너무 재미있어요 uses 이/가 to mark the book as the subject, which can feel like you’re pointing out a new or surprising fact. 은/는 (as in 이 책은 너무 재미있어요) treats the book as an established topic and comments on it. Both are correct; the nuance is subtle.