Learn Hardcore Korean: hugi bonikka johdaneun mari manha. - Looking at the reviews, there are many saying it’s good.

Both are fine. In casual speech Koreans often drop object particles (을/를) when the meaning is clear. 후기 보니까 sounds natural/conversational. 후기를 보니까 is a bit more careful/explicit. If the object is long or there’s potential ambiguity, keep the particle.

What exactly does -니까 in 보니까 mean here—“because” or “when”?

It’s the connective “when/since (I) looked,” used to set up a background for what the speaker discovered. Nuance: “Upon looking (at the reviews), I find/it turns out that…,” which sits between “when” and an explanatory “so/because.”

What’s the difference among 보니까, 보니, 봐서, and 봤더니?

보니까: Very common, conversational. “When/since I saw…” leading to a discovery or conclusion.
보니: Slightly leaner/more neutral; similar meaning.
봤더니: Past, sequential discovery. “I looked and (as a result) found….” Great with -더라.
봐서/봐서는: “Having seen/judging from.” Often used for evaluation, e.g., 후기로 봐서는 괜찮아 보여. Less natural as a simple backgrounder here.

How does 좋다는 말 work? What is -다는 doing?

It’s an indirect quote modifying a noun: “the statement/comment that (it) is good.” It’s a shortened form of 좋다고 하는 말 → 좋다는 말. This pattern is common with nouns like 말/얘기/소문/평/기사.

Why use 말 when these are written reviews?

In Korean, 말 often stands for “what people say,” whether spoken or written. It’s idiomatic to say —다는 말이 많다 even if you read it online. You can be more specific with:

좋다는 후기/평/댓글이 많아.

Could I just say 좋대 or 좋다고 해? How is that different from 좋다는 말이 많아?

좋대 (= 좋다고 해 contracted) is hearsay: “They say it’s good,” with no emphasis on quantity.
좋다는 말이 많아 highlights that there are many such comments.
You can also say 좋다고들 해 to imply “a lot of people say so.”

What register is this? How do I make it polite or formal?

The given line is casual. Politer/formal variants:

후기(를) 보니까 좋다는 말이 많아요. (polite)
후기를 보니 좋다는 말이 많습니다. (formal)
Slightly bookish: 후기를 보니 좋다는 평이 많습니다.

Who or what is “good” here? The subject of 좋다 is missing—how is that understood?

Korean drops obvious subjects. Context supplies “it” (product, place, etc.). To be explicit:

이 제품이 좋다는 말이 많아.
Or make the speakers explicit: 사람들이 좋다고 해.

Does 말이 많아 ever imply something negative like “there’s controversy”?

Yes. 말이 많다 can mean “there’s a lot of talk/controversy.” Here it’s clearly positive because the content is specified: 좋다는 말. Without that clause, 말이 많아 can sound negative or at least neutral.

Is 말들이 많아 better since we’re talking about many comments?

Use plain 말이 많아. The plural -들 isn’t used with mass/abstract nouns like 말 unless you want to strongly emphasize individuality. If you want countable items, use a countable noun: 댓글이 많아, 후기가 많아.

Can I replace 말 with something more specific?

Yes:

좋다는 후기/평/댓글이 많아.
Note the difference: 좋은 후기 means “reviews that are good (quality/positive tone),” not “reviews saying it’s good.” Use 좋다는 N to express quoted content.

Is 좋다고 말이 많아 correct? When do I use -다고 vs -다는?

Say 좋다는 말이 많아 or 좋다고 하는 말이 많아.

-다고 attaches to verbs like 하다/말하다: 좋다고 말하다/한다.
When directly modifying a noun, use the quoting modifier -다는: 좋다는 말/소문/평.

How do I change the tense inside the quoted part?

Past: 좋았다는 말이 많아 (many comments that it was good).
Future/probability: 좋을 거라는 말이 많아 (many comments that it will/probably will be good).
Habitual/action verbs: 간다는 말, 나온다는 소문, etc.

Is 보니깐 acceptable?

Nonstandard spelling. The standard is 보니까. You’ll see -니깐 online, but write -니까.

How can I emphasize that I realized this upon checking?

Use the discovery tone -더라:

후기(를) 보니까 좋다는 말이 많더라.
Or with sequential discovery: 후기를 봤더니 좋다는 말이 많더라.

What are other natural ways to say the same idea?

후기 보니까 좋다는 평이 많아.
후기 보니까 다들 좋다고들 하더라.
리뷰를 보니 평이 좋더라.
후기를 봐서는 괜찮아 보이네. (judging from the reviews)

Is 좋은 말이 많아 the same as 좋다는 말이 많아?

No. 좋은 말 = “good/kind/positive words.” 좋다는 말 = “comments that say it is good.” For food, compare 맛있는 리뷰 (a well-written “delicious review,” odd) vs 맛있다는 리뷰 (“reviews saying it’s delicious”).

Any quick pronunciation tips?

후기 보니까 ≈ “hu-gi bo-ni-kka”
좋다는 ≈ “jo-ta-neun” (the ㅎ makes a clear t sound in 좋다)
말이 많아 ≈ “ma-ri ma-na”
Natural rhythm: slight break after 보니까: “후기 보니까 | 좋다는 말이 많아.”