Learn Hardcore Korean: gyosil changmuneul yeolmyeon bakkat soriga jal deullyeoyo. - When you open the classroom window, you can hear outside sounds clearly.

-면 is the conditional suffix attached to a verb stem to express “if” or “when.” For 열다 (to open), the stem is 열, so 열 + -면 = 열면, meaning “if/when you open.” It can describe a general truth or habitual action (“When you open the window, …”) or a simple condition (“If you open the window, …”).

Why is 창문 followed by 을 in 교실 창문을 열면?

을/를 marks the direct object of a transitive verb. Here, 창문 (“window”) is what you open, so it takes 을, the object particle. Hence 교실 창문을 열면 literally means “if/when you open the classroom window.”

Why is 바깥 소리 marked with 가 instead of 를 in 바깥 소리가 잘 들려요?

The verb 들리다 is intransitive (passive-like) and means “to be heard” or “to come through (to one’s ears).” In such constructions, the thing heard becomes the subject, taking 가 (or 이), not 를. So 바깥 소리 (“outside sounds”) is the subject, giving 바깥 소리가 잘 들려요 (“outside sounds are heard well”).

What’s the difference between 들려요 and 들어요?

들어요 comes from 듣다, an active verb meaning “to listen” or “to hear” intentionally.
들려요 comes from 들리다, an intransitive/passive verb meaning “to be heard” or “to reach one’s ears” on its own.
Use 들려요 when you mean that a sound naturally comes through: “I can hear (it).”

What role does 잘 play in 바깥 소리가 잘 들려요?

잘 is an adverb meaning “well” or “clearly.” It modifies the verb 들려요, indicating the manner in which the outside sound is heard: “(you) can hear the outside sounds well/clearly.”

Why is there no explicit subject like “I” or “you” in this sentence?

Korean often omits subjects when they’re understood from context.

In 교실 창문을 열면, the implied subject of 열면 is you (“if you open…”).
In 바깥 소리가 잘 들려요, the implied subject of 들려요 is the person hearing (typically the speaker or listener).
Because context makes these clear, native speakers drop the pronouns.

Could I say 바깥 소리를 잘 들을 수 있어요 instead of 들려요?

Yes. 듣다 + -을 수 있다 (to be able to hear) yields 들을 수 있어요, so 바깥 소리를 잘 들을 수 있어요 means “you can hear the outside sounds well.”
Nuance difference:

들려요 emphasizes that the sound comes through on its own (passive).
들을 수 있어요 emphasizes the listener’s ability or opportunity to hear.

What’s the difference between 열면 and expressions like 열자마자 or 열었을 때?

All three introduce time or condition but with distinct nuances:
• 열면 (“if/when you open”) expresses a general condition or habitual fact.
• 열자마자 (“as soon as you open”) emphasizes immediacy—the next event happens right after opening.
• 열었을 때 (“when you opened”) refers to a specific moment in the past (“at the time of opening”).