Learn Hardcore Korean: chinguga chucheonhan eumak sueobeul sincheonghaesseoyo. - I applied for the music class that my friend recommended.

-가 is the subject particle in Korean. It marks 친구 as the one performing the action (recommending). If you used -는 instead (친구는), you’d be topicalizing “friend” (as in “Speaking of my friend…”), but here -가 simply indicates “friend” is the subject of 추천한.

What is 추천한, and why does it end with -ㄴ?

추천한 comes from the verb 추천하다 (“to recommend”) plus the modifier ending -ㄴ, which creates a past-tense relative clause. So 추천한 means “recommended” (in the past) and directly modifies the noun 음악 수업 (“music class”).

Could I use 추천하는 instead of 추천한?

Yes, but the nuance changes.

추천하는 음악 수업: “the music class that (my) friend is recommending” (present/ongoing).
추천한 음악 수업: “the music class that (my) friend recommended” (completed action in the past).

Why do we say 음악 수업 instead of just 수업?

음악 means “music,” and 수업 means “class” or “lesson.” Together, 음악 수업 specifies the subject of the class (“music class”). Without 음악, 수업 could refer to any kind of lesson.

Why is 음악 수업 followed by 을?

을/를 is the object particle. 음악 수업 is what you applied for, so it takes 을 to mark it as the direct object of the verb 신청했어요.

Why does the verb 신청했어요 come at the end of the sentence?

Korean follows Subject–Object–Verb (SOV) word order. The verb always comes last, after both the subject (친구가 추천한 음악 수업을) and any objects or modifiers.

Can I say 음악 수업에 신청했어요 instead?

No. 신청하다 is a transitive verb that takes a direct object with -을/를. Using -에 (the location/time particle) here would be ungrammatical for “apply for a class.”

What kind of verb is 신청하다, and how is it used?

신청하다 is a compound verb made of the Sino-Korean noun 신청 (“application”) plus 하다 (“to do”). It means “to apply for” or “to register for.” You attach -을/를 to the thing you’re applying for (e.g., 음악 수업을 신청하다).

Why is the ending -했어요, and what level of politeness does it convey?

-했어요 is the polite informal past-tense form of 하다.

했어요 = “(I) did (something)” in a polite but casual register.
For more formality you’d use 신청했습니다, and for close friends/casual speech you might say 신청했어.

What is a relative clause in Korean, and how does 친구가 추천한 function as one?

In Korean, relative clauses come directly before the noun they describe, with no separate word like “that” or “which.” You form them by adding a modifier ending (e.g., -는, -ㄴ/은, -을) to the verb stem.

친구가 추천한 = “that (my) friend recommended”
This entire phrase modifies 음악 수업, so 친구가 추천한 음악 수업 literally means “the music class that my friend recommended.”