Learn Hardcore Korean: chaegeul ilgdaga jollyeoseo jamdeureosseoyo. - I was reading a book and got sleepy, so I fell asleep.

The ending -다가 attaches to verb stems to indicate that one action was interrupted by another or that something happened in the middle of doing it. Here, 읽다 (to read) + -다가 shows “while reading (the book), ….” It implies that the reading was ongoing when the next event occurred.

Why is 책을 marked with -을 instead of -를?

Both -을 and -를 are object particles in Korean; -을 follows nouns ending in a consonant, while -를 follows nouns ending in a vowel. 책 ends in the consonant ㄱ, so its object form is 책을.

What role does 졸려서 play in this sentence?

졸려서 is the connective form of 졸리다 (to feel sleepy). When you change -아/어 to -아/어 서, it indicates cause or reason: “because I was sleepy.” So 읽다가 졸려서 means “while I was reading, I got sleepy, and as a result ….”

Why is 잠들었어요 used instead of simply 잤어요?

잠들다 literally means “to fall asleep,” emphasizing the moment of drifting off. 잤어요 means “I slept.” In this sentence, the focus is that reading led directly to falling asleep, so 잠들었어요 is more precise.

Could you say 읽고 잠들었어요 instead? How is that different from 읽다가?

Yes, 읽고 is another connective (from 읽다 + -고), meaning “after reading.” It suggests you finished reading first, then fell asleep. 읽다가 emphasizes that the sleepiness occurred in the middle of reading and interrupted it.

Why isn’t there a subject like “I” or “she” in the sentence?

Korean often omits the subject when it’s clear from context or from who’s speaking. Here, it’s understood that the speaker (“I”) is the one reading and falling asleep, so no explicit 저는 or 제가 is needed.

Can the order of the clauses change? For example, 졸려서 책을 읽다가 잠들었어요?

That reordering would sound odd. Korean connectives generally flow from the first action/context to the result: (Action A + -다가) + (Reason B + -서) + (Result C). Swapping B and A breaks the natural causal sequence and is ungrammatical here.