Learn Hardcore Korean: goyangineun changmun yeopeseo jago isseo.

은/는 marks the topic. 고양이는 means “as for the cat…,” assuming the cat is already known or you’re contrasting it with something else. 고양이가 (subject marker) would emphasize who is sleeping (e.g., in answer to “Who’s sleeping?”). Both are grammatically fine; the nuance differs.

Can I drop 는 or even the whole subject?

Yes. In casual speech you can say 고양이 창문 옆에서 자고 있어 or, if the subject is obvious, just 창문 옆에서 자고 있어. Dropping is natural when context makes the subject clear.

Why 창문 옆에서 and not 창문 옆에?

Use 에서 for where an action happens. 자다 is treated as an activity, so 창문 옆에서 자고 있어. Use 에 for static existence: 창문 옆에 고양이가 있어 = “There’s a cat next to the window.”

Why attach 에서 to 옆 instead of to 창문?

옆 is a relational noun (“side of”). The location particle attaches to it: [명사] + 옆 + 에서 → 창문 옆에서. 창문에서 means “at the window” (not specifically next to it), which is a different nuance.

Should it be 창문의 옆에서 with 의?

Normally no. With relational nouns like 옆, 앞, 뒤, 위, 아래, 사이, native usage typically drops 의. 창문 옆에서 sounds natural; 창문의 옆에서 sounds stiff or bookish.

What does 자고 있어 mean exactly, and how is it different from 자고 있다?

Both express the progressive “be -ing.” 자고 있어 is casual plain; 자고 있다 is the base form and also fine in narrative/written style. Polite versions are 자고 있어요 and 자고 있습니다. Meaningwise, they’re the same.

Why does 있다 show the progressive here when it usually means “to exist”?

In the construction -고 있다, 있다 functions as an auxiliary marking ongoing action (grammaticalized usage). So 자고 있어 = “is sleeping,” 먹고 있어 = “is eating,” etc. It’s not the existential meaning in this pattern.

Are there other natural ways to say “is sleeping,” like 자는 중 or 잠들어 있어?

자는 중(이야/이에요): “in the middle of sleeping,” slightly more deliberate/temporary; common on the phone: 지금 자는 중이에요.
잠들어 있어: “is asleep” (result state after 잠들다 “to fall asleep”).
Habitual/generic: 고양이는 창가에서 잘 잔다 = “Cats sleep well by windows.”

How do I make the sentence polite or formal?

Polite: 고양이는 창문 옆에서 자고 있어요.
Formal: 고양이는 창문 옆에서 자고 있습니다.
The rest of the sentence stays the same; only the ending changes.

Can I move the location phrase around?

Yes. Default is [Topic/Subject] + [Place] + [Verb]: 고양이는 창문 옆에서 자고 있어.
For emphasis you can say 창문 옆에서 고양이는 자고 있어 (emphasizes the place). Putting the place after the verb (…자고 있어, 창문 옆에서) is colloquial and sounds tacked on.

Are 창문 옆 and 창가 the same?

Close. 창가 means “the area by the window” (often for seats/edges) and is very idiomatic: 창가에서. 창문 옆 literally “beside the window.” Both work here; 고양이는 창가에서 자고 있어 sounds very natural. You may also see literary 창문 곁에서 or broader 창문 근처에서 (“near the window”).

Why isn’t there an object marker like 창문을?

자다 is intransitive (no direct object), so there’s nothing to mark with 을/를. 창문 is part of a location phrase, not an object.

Does 고양이는 mean “the cat” or “a cat”? How do I make it explicit?

Korean has no articles, so context decides. 고양이는… can be “the cat” (known one) or introduce “a cat.” To be explicit:

“A cat” (new): 고양이 한 마리가 창문 옆에서 자고 있어.
“That/the specific cat”: 그 고양이는 창문 옆에서 자고 있어.

What’s the correct spacing? I’ve seen 자고있어 and 창문옆에서 online.

Standard spacing is: 창문 옆에서 자고 있어.

자고 있어 (auxiliary 있다 is spaced)
창문 옆 (relational noun 옆 is spaced)
Writing 자고있어, 창문옆에서 is common informally online but nonstandard.

Any quick pronunciation tips?

옆에서: the final ㅍ of 옆 links to the vowel, so it sounds like “yeo-pe-seo.”
자고 있어: pronounce the ㅆ in 있어 as a clear double s: “ja-go iss-seo.”
Keep a smooth link between words; there’s no strong pause inside the phrase.

How do I make it a question or negate it?

Yes–no question (casual/polite/formal): 고양이는 창문 옆에서 자고 있어? / …자고 있어요? / …자고 있습니까?
Negation (casual/polite/formal): 자고 있지 않아. / 자고 있지 않아요. / 자고 있지 않습니다.