Learn Hardcore Korean: oneul cheoeum mannan seonsaengnimi jeongmal chinjeolhaesseo. - The teacher I met for the first time today was very kind.

이/가 marks the grammatical subject; here, 선생님이 = “the teacher (as the subject)”.
은/는 marks the topic/contrast. 선생님은 정말 친절했어 frames “the teacher” as the topic (possibly contrasting with others).
Subtlety: 이/가 often presents new/focused info; 은/는 often sets context or contrasts.

How does the chunk 오늘 처음 만난 선생님 work?

It’s a relative clause: “[the teacher] whom (I) met for the first time today.”
Order: modifier before noun. 오늘 (today) + 처음 (for the first time) + 만난 (met, attributive past) + 선생님 (teacher).
Korean drops the internal subject “I,” which is understood from context.

Why 만난 and not something like 만났?

To modify a noun with a past action, Korean uses the attributive past -(으)ㄴ.
만나다 → stem 만나-
- -ㄴ → phonologically becomes 만난.
More examples: 읽다 → 읽은 책 (the book I read), 먹다 → 먹은 밥 (the rice I ate).

What’s the difference among 만난 / 만나는 / 만났던 / 만날?

만난 선생님: the teacher I met (completed past event).
만나는 선생님: the teacher (I am/was in the process of) meeting; ongoing/habitual.
만났던 선생님: the teacher I had met before; past experience with a sense of “back then” or discontinuity.
만날 선생님: the teacher (I will meet / am about to meet); prospective.

Why is there no object marker 을/를 inside 처음 만난 선생님?

In Korean relative clauses, the head noun (선생님) corresponds to a “gap” inside the clause, so you do not mark it with 을/를 there.
Think: “(I) met _ for the first time today” → that blank is 선생님.
If you want an explicit object marker, you’d restructure as two clauses: 오늘 선생님을 처음 만났는데, 그분이 정말 친절했어.

Is 친절했어 polite enough when talking about a teacher?

-었어 is casual (to friends/peers). More appropriate forms:
- Polite: 친절했어요
- Polite + honorific for the teacher: 친절하셨어요
- Formal: 친절했습니다
Full polite/honorific rewrite: 오늘 처음 만난 선생님께서 정말 친절하셨어요.

Should I use 께서 instead of 이 for a teacher?

께서 is the honorific subject marker; it’s respectful for people of higher status.
선생님께서 정말 친절하셨어요 is very natural and respectful.
In casual talk to friends, 선생님이 … 친절했어 is fine, but it’s less deferential.

Would it be better to say 뵙다 instead of 만나다 for a teacher?

뵙다 is the humble verb “to meet/see (a respected person)” from the speaker’s perspective.
Honorific version: 오늘 처음 뵌 선생님께서 정말 친절하셨어요.
Avoid the redundant hybrid 만나 뵙다 in this context; just use 뵙다.

What’s the nuance of 정말? How does it compare to 진짜/아주/매우/너무/되게?

Can I say 오늘은 instead of 오늘? What changes?

오늘 is a plain time adverb: “today.”
오늘은 topicalizes “today,” often implying contrast or setting up today as the frame: “As for today, the teacher I met for the first time was really kind (perhaps unlike other days).”
Both are grammatical; choose based on whether you want to highlight/contrast “today.”

Why not 첫 만난 선생님? What’s the difference among 처음 / 첫 / 첫 번째 / 처음으로?

처음 만난 = met for the first time (ever). Natural.
첫 is an attributive adjective: use it with nouns like 첫 만남 (the first meeting), 첫사랑 (first love). 첫 만난 is unnatural.
첫 번째 is ordinal “first (number 1)”: 첫 번째로 만난 선생님 = the teacher I met first (among several today).
처음으로 is adverbial “for the first time”: 오늘 처음으로 만난 선생님 (a bit heavier than 처음 만난).

Does 오늘 처음 만난 선생님 mean “the first teacher I met today” or “the teacher I met for the first time today”?

By default, it means “the teacher I met for the first time today (ever).”
To mean “the first teacher I met today (out of several),” say 오늘 제일 먼저 만난 선생님 or 오늘 첫 번째로 만난 선생님.

Where can I place 정말? Is the current position the only option?

Adverbs are flexible. Common placements:
- … 선생님이 정말 친절했어. (most natural)
- 정말 … 친절했어. (emphasizes the whole statement)
Don’t split the relative clause; keep 오늘 처음 만난 together before 선생님.

Why 친절했어 and not something like 친절이었다?

친절하다 is a descriptive verb (adjective-like verb). Past tense is 친절했어/습니다, not 친절이었다, which sounds odd here.
Pattern: [stem]하다 → [stem]했- in past (e.g., 행복했어, 간단했어요).

How would I say it if I haven’t met the teacher yet?

Use a prospective modifier: 오늘 처음 만날 선생님은 정말 친절해 보이셔. (The teacher I’ll be meeting for the first time today seems really kind.)
Or present/ongoing: 오늘 처음 만나는 선생님은 …

How can I add “to me” (e.g., “was really kind to me”)?