Learn Hardcore Korean: naega da-eume ilgeul chaegeul gollasseo.

Korean pronouns need particles to show their role. 가 is the subject marker, so 나 + 가 = 내가 marks “I” as the subject. 나 alone isn’t enough, and 나는 (with the topic marker 는) would make “I” the topic, implying contrast or known information. Here, 내가 simply identifies who chose the book without added nuance.

How does 다음에 읽을 책 work? Why is 읽을 placed before 책?

Korean verbs can directly modify nouns by taking their attributive form. 읽다 (to read) becomes 읽을, meaning “that will be read.” Placing 읽을 before 책 yields 읽을 책 (“the book that [I] will read”). Adding 다음에 (“next time”) in front gives 다음에 읽을 책 (“the book that I will read next”).

Why isn’t there a future auxiliary like -거예요 with 읽을?

When a verb modifies a noun, you use the -(으)ㄹ ending on the verb stem to indicate future action in an attributive clause. You do 읽다 + ㄹ → 읽을 instead of using 읽을 거예요, because -거예요 is a predicate ending, not a noun‐modifier.

Why does 책 have the object marker -을 in 책을 골랐어 after already being in 읽을 책?

The entire phrase 다음에 읽을 책 functions as the object of 골랐어 (“chose”). In Korean, objects take 을/를, so you add -을 to the noun phrase: 읽을 책을 골랐어 (“I chose [the book that I will read next]”).

Can I insert 내가 inside the noun phrase itself, like 다음에 내가 읽을 책?

Yes. Korean relative clauses are flexible. 다음에 내가 읽을 책 still means “the book that I will read next.” Putting 내가 before 읽을 makes it explicit who does the reading in the clause.

What speech level is 골랐어, and how do I make it polite?

골랐어 is informal (반말). To be polite, use 골랐어요. For a formal style, say 골랐습니다 and change 내가 to the honorific 제가.

Is it natural to move 다음에 somewhere else, like after 책을?

Adverbs like 다음에 are flexible but usually placed before the verb or at the start. 다음에 읽을 책을 골랐어 sounds most natural. Moving it after 책을 (e.g., 책을 다음에 골랐어) would change the meaning (“I chose the book later/next”) and feel awkward.