Learn Hardcore Korean: geuneun joheun sosigi issdago haesseo.

It marks indirect/reported speech: “said that ….” Here, 있다 + 고 하다 → 있다고 하다, and 했어 puts “said” in the past, casual form. So the sentence reports what he said without directly quoting him.

Why is the particle 이 used on 소식 (소식이), not 을/를?

With 있다/없다 (exist/there is/there isn’t), the thing that exists is marked with the subject marker 이/가. So you say 소식이 있다 (there is news), not 소식을 있다.

Why 좋은 소식 and not 좋다 소식 or 소식이 좋다?

좋다 is a predicate (“to be good”). Its adnominal (prenominal) form is 좋은, which modifies a noun: 좋은 소식 = good news.
좋은 소식이 있다고 했어 = He said there is good news.
소식이 좋다고 했어 = He said the news is good (commenting on specific news). Different meaning.

What’s the difference between 그는 and 그가 here?

그는 (topic): sets “he” as the topic; neutral.
그가 (subject/focus): emphasizes “HE (as opposed to someone else) said ….” Both are grammatical:
그는 좋은 소식이 있다고 했어. (neutral)
그가 좋은 소식이 있다고 했어. (contrastive/emphatic)

Is 그는 natural in everyday Korean? How else would people say “he”?

In casual speech, people often avoid 그는/그가.

Omit the pronoun if clear: 좋은 소식이 있대.
Use a name/title: 민수 씨가 좋은 소식이 있대., 팀장이…
Use 그 사람, 걔 (informal), depending on context.

What politeness/tense level is 했어, and what are alternatives?

했어: past, casual.
했어요: past, polite.
했습니다: past, formal.
했다: past, plain/written. Example: 그는 좋은 소식이 있다고 했어요/했습니다/했다.

Can I use 말하다 instead of 하다?

Yes: 그는 좋은 소식이 있다고 말했어/말했어요. It explicitly means “said.” -고 하다 is the default, very common reporting construction.

When do I use -다고 vs -라고 in reported speech?

Use -다고 하다 after verbs/adjectives and 있다/없다: 간다고 했어, 춥다고 했어, 있다고 했어, 없다고 했어.
Use -(이)라고 하다 after nouns with 이다/아니다 and for direct quotes: 학생이라고 했어; direct quote: 그는 내일 갈게라고 했어.

How do I adjust the tense inside the quoted clause?

Present in the quote: 있다고 했어 (He said there is).
Past in the quote: 있었다고 했어 (He said there was).
Future prediction: 있을 거라고 했어 (He said there would be). If he is saying it now: 있다고 해/있다고 해요.

How do I make this honorific/polite when talking about someone respected?

Honorify the reporting verb: 그분이 좋은 소식이 있다고 하셨어요/하셨습니다.
You can also say: 그분이 … 있다고 말씀하셨어요.
Don’t change 있다 to 계시다 here, because the thing that “exists” is 소식 (not a person).

Any pronunciation tips for this sentence?

있다고 ≈ ittago (ㅆ + ㄷ sounds like a tense tt).
좋은 ≈ joeun (ㅎ is weak/silent here).
소식이 ≈ soshigi (liaison).
했어 ≈ haesseo (often sounds like hae-sso).

Can I change the word order?

Yes, but the neutral order is subject/topic first. 좋은 소식이 있다고 그는 했어 is possible but sounds marked/emphatic. It’s common simply to drop 그는 if context is clear.

Can I drop particles like 이 or 는?

Dropping 그는 is common when context is clear.
You can drop 이 in speech: 좋은 소식 있다고 했어. In writing or when clarity matters (especially with 있다/없다), keep 이/가.

What about the contraction -대 I hear in speech?

그는 좋은 소식이 있대. = He says there is / I hear there is good news.
Polite: 있대요.
Past/future: 있었대, 있을 거래(요) (from 있을 거라고 해(요)).
Common error: not 있데, but 있대.

Can I say 좋은 소식은 있다고 했어? What changes?

Yes, but 은/는 adds contrast/topic: 좋은 소식은 있다고 했어 implies “At least as for good news, there is (maybe other kinds aren’t).” For a neutral statement, use 이/가: 좋은 소식이 있다고 했어.

How do I negate or ask about it using this pattern?

Negative: 그는 좋은 소식이 없다고 했어. (He said there isn’t any good news.)
Asking (reported question): use -냐고 with a verb like 묻다/물어보다. For example, “He asked if there was good news” → 그가 좋은 소식이 있냐고 물었어.