Learn Hardcore Korean: naneun jihacheoreseo cheoncheonhi chaegeul ilgeo.

-에서 marks the location where an action happens (“at,” “in,” or “on” in English). 지하철에서 therefore means “on the subway” or “in the subway.” If you used -에, it would point to a destination or state (“to the subway” or “at the subway” in a static sense) rather than the place of the action.

Why is 책을 used instead of just 책?

In Korean, direct objects take the object marker 을 (after a consonant) or 를 (after a vowel). Since 책 ends in the consonant ㄱ, you attach 을 to show “book” is what’s being read. Without 을, the sentence would sound incomplete.

Why is the verb 읽어 in this form? What tense, aspect, and politeness level is it?

읽어 is: • Present tense (it can be simple present or imply ongoing action)
• Declarative mood (stating a fact)
• Informal, low speech level (반말)

It’s the plain casual form you’d use with close friends or oneself.

Can I express the continuous action “reading” more explicitly by saying 읽고 있어?

Yes. 읽고 있어 (e.g. 읽고 있어요 in polite form) uses the progressive “to be reading.” But in everyday casual Korean, the simple present 읽어 often conveys “I am reading” just fine, especially when the context (like “on the subway”) already implies it’s happening right now.

What role does 천천히 play, and can I move its position?

천천히 is an adverb meaning “slowly,” and it modifies the verb 읽어. In Korean, adverbs usually appear right before the verb they describe. You generally wouldn’t insert something between 천천히 and 읽어, though you could place 천천히 elsewhere for emphasis (e.g. at the very start), but the natural spot is directly before the verb.

Why is 나는 used at the beginning? Can it be omitted?

나는 is the topic-marked pronoun “I am.” In Korean, subjects/topics are often omitted when they’re clear from context. If you’ve already established who’s doing the action, you can say 지하철에서 천천히 책을 읽어 and it will still mean “I read a book slowly on the subway.”

How would you make this sentence polite or formal?

To speak politely you can:
• Change 읽어 to 읽어요 and use 저는 instead of 나는:
저는 지하철에서 천천히 책을 읽어요.
To be formally polite (e.g. in a presentation or announcement):
저는 지하철에서 천천히 책을 읽습니다.

Is there a nuance difference between using 나/“나는” and 저/“저는”?

Yes. 나 is an informal first-person pronoun used with people you’re close to. 저 is humble/formal and used in polite or respectful situations. The particles 는 attach in the same way, but choosing 나 vs. 저 signals your level of formality.