Learn Hardcore Korean: i chaegeun eolmayeyo?

은 is the topic marker attached to a noun ending in a consonant (here 책). By saying 이 책은…, you’re setting “this book” as the topic or theme of your sentence: “As for this book, how much is it?”

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

은/는 marks the topic or what you’re talking about, often implying contrast or general statements.
이/가 marks the grammatical subject, used for identification or emphasis.
So 이 책은 얼마예요? asks about the price of this book in general, while 이 책이 얼마예요? focuses more tightly on “this book” as the subject of the verb.

Why is it 예요 and not 이에요, and when do I use each?

The copula has two polite forms:

예요 follows words ending in a vowel.
이에요 follows words ending in a consonant.
Since 얼마 ends in the vowel ㅏ, you attach 예요 → 얼마예요.

What’s the difference between 얼마예요 and 얼마입니까?

Both mean “How much is it?” but differ in formality:

얼마예요? is polite but casual (요-form), suitable for shops, classmates, coworkers.
얼마입니까? is more formal/deferential (합니다-form), used in official announcements or very formal situations.

Why do we say 얼마예요 instead of using a verb like “to cost”?

Korean asks for price with the question word 얼마 plus the copula -이다 (turned into 예요 here). There is no dedicated “cost” verb in price questions.
If you want to ask “Is it expensive?”, you’d use 비싸다: 이 책이 비싸요?
But to get the exact price, always use 얼마예요?

How would I respond to 이 책은 얼마예요? with the price?

State the amount, add the currency counter, then the copula:

8,000원이에요.
15달러예요.
20유로예요.

Can I use 이거는 얼마예요? instead of 이 책은 얼마예요?

Yes. 이거 means “this (thing).” If you’re pointing at the book, 이거는 얼마예요? is perfectly natural and a bit more casual.

How can I make it more casual among close friends?

In very informal speech you can drop the topic marker and switch to the casual ending -야:

이 책 얼마야?
But be careful: this is only for close friends or younger people.