Learn Hardcore Korean: iburi neomu dukkeowoseo yeoreume deowoyo. - The blanket is too thick, so it’s hot in the summer.

이불 is the Korean word for the bedding you pull over yourself at night—essentially a quilt, duvet, or comforter. In Korea it often refers to a thick, padded blanket used on a traditional floor mattress or bed.

Why is it 이불이 and not 이불은?

The particle -이 here is the subject marker (attached because 이불 ends in a consonant). It simply indicates that 이불 is the subject of 두꺼워서 (“because the blanket is thick”). Using -은 (the topic marker) would shift the nuance slightly—to “as for the blanket…”—but -이 is more neutral for stating a subject.

What function does -어서 in 두꺼워서 serve?

The ending -어서 connects two clauses and expresses cause or reason. So 두꺼워서 literally means “because (it is) thick.” It explains why the second clause (여름에 더워요) happens.

Why does 두껍다 change to 두꺼워서 instead of 두껍어서?

Because 두껍다 is a ㅂ-irregular adjective. When a ㅂ-ending stem meets a vowel-initial ending, the ㅂ changes to 우, so:
• 두껍 + 어서 → 두꺼우 + 서 → 두꺼워서.

What nuance does 너무 have in 이불이 너무 두꺼워서? Does it mean “too” or “very”?

Here 너무 means “too (much)”, indicating an excessive degree that causes a problem. So 너무 두꺼워서 = “because it’s too thick.” (Note: in casual speech, 너무 can sometimes mean “very,” but when paired with -어서 it usually carries the “too” nuance.)

Why is 여름에 marked with -에, and could we say 여름에는 instead?

-에 marks a time adverbial (“in/at summer”). You could use 여름에는 to add a contrastive or topical nuance—“as for summer”—but the basic 여름에 더워요 simply states “it’s hot in summer.”

Why is there no explicit subject for 더워요? Whose “hot” is it?

Korean often omits the subject when it’s clear from context. Here 더워요 (“it’s hot”) implies “I” (or “we”) feel hot because of the thick blanket. Translating it: “Because the blanket is too thick, (I) feel hot in summer.”

Can we rearrange the sentence to 여름에는 이불이 너무 두꺼워서 더워요? Would that sound more natural?

Yes. Putting 여름에는 first is common:
• 여름에는 이불이 너무 두꺼워서 더워요.
This emphasizes “in summer” as the topic while conveying the same meaning.