Learn Hardcore Korean: seonsaengnimi geureohdago haesseoyo.

그렇다 means “to be so/that way.”
Add the indirect-quote marker -다고 (used for declarative content with descriptive adjectives) to get 그렇다고, meaning “that it is so,” “that’s the case,” or simply “so.”
So 선생님이 그렇다고 했어요 = “The teacher said that it’s so” → “The teacher said so.”

Why is the subject marked with 이 (선생님이) instead of 은/는?

이/가 marks who actually performed the action (narrow focus): here, who said it? → the teacher.
은/는 makes the noun a topic and often adds contrast: 선생님은 그렇다고 했어요 can imply “As for the teacher (as opposed to others), (he/she) said so.”
Both are grammatically fine; choose 이/가 for neutral identification, 은/는 for contrast or topic shift.

Why 했어요? Could I use 말했어요 instead?

-다고 하다 is the standard pattern for reporting speech; 하다 is a light verb here meaning “say.”
선생님이 그렇다고 했어요 and 선생님이 그렇다고 말했어요 are both correct. The version with 하다 is more common and slightly lighter.
If you want to highlight the act of speaking, 말했어요 is fine. In everyday speech, 했어요 with the quotative is very natural.

How would I make this more honorific since it’s about a teacher?

Use subject honorifics:

선생님께서 그렇다고 하셨어요.
Or use the honorific verb for speaking: 선생님께서 그렇다고 말씀하셨어요. Both are polite and respectful. Adding -께서 is the honorific subject marker.

Does this sentence mean “The teacher agreed”?

It often does, depending on context. If someone asked for permission or confirmation, 그렇다고 했어요 can imply “(The teacher) said yes/that’s okay.”
If you want to be explicit, quote the specific content:
- Permission: 선생님이 해도 된다고 하셨어요 (The teacher said it’s allowed).
- Confirmation: 맞다고 하셨어요 (He/She said it’s correct).

Why is it -다고 and not -라고?

Use -다고 for declarative indirect quotes with verbs/adjectives like 그렇다, 예쁘다, 가다 (with the right present marker).
Use -(이)라고 when the predicate is a noun with 이다 or for direct quotes. Examples:
Descriptive adjective: 예쁘다고 했어요 (said it’s pretty).
Action verb (present): 간다고 했어요 (said [someone] is going).
Noun + 이다: 학생이라고 했어요 (said [someone] is a student).
Direct quote: “그래”라고 했어요.

How does this differ from the direct-quote style?

Indirect (your sentence): 선생님이 그렇다고 했어요 (paraphrases the content).
Direct: 선생님이 “그래”라고 했어요 (quotes the exact wording). Indirect is more common in narration; direct preserves exact wording and level/style.

Can I just say 그랬어요 instead?

그랬어요 is the past of 그러다 (to do so/that way) or 그렇다 (to be so), and can mean “It was like that” or “(He/She) did so.”
It does NOT explicitly mean “said so.” To report speech clearly, keep -다고 했어요.

Is the word order fixed? Can I say 그렇다고 선생님이 했어요?

Natural patterns:
- 선생님이 그렇다고 했어요 (most natural).
- 그렇다고 했어요, 선생님이 (possible for afterthought/emphasis).
그렇다고 선생님이 했어요 is possible but sounds awkward in simple statements. Put the subject before the quote or leave it after as an emphatic tail.

What tense/nuance does 했어요 give? How would I say “He says so (now)” or “I heard he said so”?

했어요 = past reporting: “(He/She) said (earlier).”
Ongoing/current report: 그렇다고 해요 → colloquial contraction 그렇대요 (“He/She says so”).
Hearsay/reporting what you heard: 그렇다고 했대요 or 그랬대요 (“I hear he/she said so”).

Any pronunciation tips for 그렇다고?

It’s pronounced close to [그렇타고]. The ㅎ influences the following ㅌ sound, so you’ll hear a clear [t] sound at the -타- part.

What’s the difference between 그렇다고 했어요 and 그렇다고요?

그렇다고 했어요: neutral report—“(Someone) said so.”
그렇다고요?: rising intonation as a question—“Did you say it’s so?” or “Is that what you said?”
그렇다고요. (falling intonation): assertive reiteration—“I said it is so.” It’s used to insist or remind.