Learn Hardcore Korean: i chaegi jinjja jaemiisseoyo. - This book is really interesting.

The first 이 is a demonstrative determiner meaning “this.”
The second 이 is the subject particle (attached to 책) that marks 책 as the grammatical subject of the sentence.
They look identical in Hangul but serve different functions.

What’s the difference between 이 책이 and 이 책은?

Both mark 이 책 (“this book”), but with different particles:

이 책이 uses 이 (subject marker) to introduce or emphasize the subject—often when giving new information or stressing what’s interesting.
이 책은 uses 은 (topic marker) to set 이 책 as a known topic or contrast it with something else.
Example: • 이 책이 재미있어요. (This book is interesting [and I’m focusing on this book as the subject].)
• 이 책은 재미없어요. (As for this book, it’s not interesting [perhaps compared to others].)

What part of speech is 진짜, and is it formal or informal?

진짜 is an adverb meaning “really,” “truly,” or “for real.”
It’s colloquial and friendly—common in spoken Korean and casual writing.
In very formal or polite contexts (e.g., academic papers, business letters), you’d usually use 정말 or 매우 instead.

Can I replace 진짜 with 정말 or 매우?

Yes. All three intensify the adjective, but with slight nuance and register differences:

진짜 재미있어요. (Very casual, conversational tone.)
정말 재미있어요. (Neutral politeness—safe in most contexts.)
매우 재미있어요. (Quite formal—common in writing or official speech.)

Why does 진짜 come before 재미있어요 instead of after?

In Korean, adverbs (like 진짜, 정말, 매우) generally precede the verb or descriptive verb they modify. Placing them after would be ungrammatical: • Correct: 진짜 재미있어요.
• Incorrect: 재미있어요 진짜.

How is 재미있어요 formed?

재미있어요 comes from:

재미 (noun: “fun,” “interest”)
있다 (descriptive verb: “to exist,” “to have”)
When combined (재미 + 있다), you get 재미있다, meaning “to be interesting” or literally “to have fun/interest.” Conjugated politely: 재미있어요.

Is 재미있어요 a verb or an adjective?

In Korean grammar, 재미있다 is a descriptive verb (often called an adjective in English explanations).

Like action verbs, descriptive verbs end in -다 in dictionary form.
They describe a state or quality rather than an action.
Conjugation patterns for descriptive verbs mirror those of action verbs in speech levels (e.g., -어요, -습니다, -아/어).

What politeness level is -어요 in 재미있어요, and how can I change it?

-어요 is the polite informal speech level (해요체), used in everyday conversations with strangers, colleagues, or acquaintances.
To change register: • More casual (반말): 이 책 진짜 재미있어.
• More formal polite (합쇼체): 이 책이 진짜 재미있습니다.

Why do I sometimes see 재밌어요 instead of 재미있어요?

재밌어요 is a very common colloquial contraction of 재미있어요:

재미 + 있
- 어요 → 재밌어요
  Both are correct and mean the same thing; 재밌어요 simply reflects everyday spoken Korean.

Can I drop 이 책이 when speaking?

Yes. Korean often omits subjects or objects when they’re clear from context. In casual speech, you might just say: • 진짜 재미있어요!
Especially if you’ve just mentioned 이 책 or are pointing at it, the listener will understand you’re talking about “this book.”