Learn Hardcore Korean: 12woreneun keuriseumaseuga isseoseo gibuni johayo. - In December, because there’s Christmas, I feel happy.

The ending -에는 combines the time/location particle -에 with the topic particle -는.
• -에 marks “in December” as a time.
• -는 then highlights December as the topic (“as for December…”), implying a contrast or special remark about that month.

Why is December written and pronounced 십이월 (12월) rather than 열두월?

Months in Korean always use Sino-Korean numerals, not native Korean numbers.
• “12” in Sino-Korean is 십이 (sip-i).
• Native Korean “twelve” is 열두, but you never say 열두월; instead you always say 십이월.

Why does 크리스마스가 take the subject marker -가 here, instead of the topic marker -는?

When you use the verb 있다 (“to exist” or “to have something”), you typically mark the thing that exists with -가 (subject marker).
• 크리스마스가 있어서 literally “because Christmas exists/occurs…”
• If you used -는, you’d be making Christmas the topic rather than the grammatical subject of 있다.

How does 있어서 function in 크리스마스가 있어서? Is it just “have” + “and”?

Here 있다 (“to exist”) is combined with the connective ending -아서/어서, which expresses cause or reason.
• 크리스마스가 있어서 = “because (there is) Christmas…”
• It’s not simply listing events (“…and”), but giving a reason for the next clause.

Could you use 크리스마스 때문에 instead of 크리스마스가 있어서? What’s the nuance?

Yes, 때문에 also expresses cause: 크리스마스 때문에 기분이 좋아요.
Nuance differences:
• -아서/어서 feels more natural and conversational, often used in spoken Korean.
• 때문에 is a bit more formal or emphatic (“due to…”). Both are correct, but the original sounds lighter and colloquial.

What is 기분 and why do we add -이 to make 기분이 좋아요?

기분 (gi-bun) is a noun meaning “feeling” or “mood.”
• -이 is the subject marker used after a noun ending in a consonant.
• 기분이 좋아요 = “(My) mood is good” / “I feel good.”

Why does the sentence end with 좋아요 and not 좋습니다? What level of speech is that?

좋아요 is the polite informal ending (합쇼체 with -요). You’d use it in everyday conversation with strangers or acquaintances.
• 좋습니다 is the polite formal ending (합쇼체 without -요), more common in presentations, announcements, or very formal settings.
Choose based on how polite or formal you want to sound.

Could we swap the clauses—say 기분이 좋아요, 크리스마스가 있어서—or is the original word order important?

Korean is relatively flexible, but the cause-effect pattern X-아서/어서 Y usually places the reason first (크리스마스가 있어서) and the result next (기분이 좋아요). Swapping them can sound awkward or like you’re emphasizing the result before the cause, which isn’t the natural flow here.