Learn Hardcore Japanese: kouen ni oozei no hito ga ite nigiyaka desu. - There are many people in the park, so it is lively.

大勢の人 = a lot of people / many people
たくさんの人 = many people (more general, can be used for things too: たくさんの本 “many books”)
多くの人 = many people (slightly more formal / written)

Nuance:

大勢 often suggests a crowd or group feeling.
たくさん is more neutral and very common.
多く feels more formal, like essays, news, or reports.

You normally say:

大勢の人が〜 (natural)
Not:
✕ 大勢が〜 (feels incomplete; you almost always say 大勢の人 or something similar).

Why is the particle に used after 公園 instead of で?

Both に and で can appear after places, but they have different functions:

に after a place often marks:
- location of existence: where something/someone is
- destination: where someone goes to
で marks the place where an action happens.

In this sentence:

公園に大勢の人がいる
→ Focus: people exist / are present in the park.

If you used 公園で, it would sound more like:

公園で大勢の人が遊んでいる
→ “A lot of people are playing in the park” (place of activity).

So with いる (“to exist / to be (for animate things)”), に is the natural particle:
公園に大勢の人がいる = “There are many people in the park.”

What is the role of いて here? Why not just say います?

いて is the て-form of いる.

いる → dictionary form
います → polite, non-past
いて → て-form (linking form)

In this sentence:

大勢の人がいて賑やかです。

The て-form is used to connect two parts:

大勢の人がいる – “There are many people”
賑やかです – “(It) is lively”

So the structure is:

(There) are many people, and (so it) is lively.

If you said it as two separate sentences:

公園に大勢の人がいます。賑やかです。

That is also correct, but using いて makes it one smooth, natural sentence and shows the causal relationship more clearly: it’s lively because many people are there.

Does いて here mean a continuous action, like “are being”? Is this a progressive form?

No. In this context, いて is not a progressive like English “are being”.

The progressive in Japanese is 〜ている, e.g. 遊んでいる “are playing”.
Here, the verb is いる itself (to exist), and いて is just its て-form used for linking clauses.

So:

大勢の人がいる = “Many people are (there).”
大勢の人がいて賑やかです = “Many people are there, and (so) it is lively.”

It’s not “people are being” anything; it just states their existence and connects that fact to 賑やかです.

What kind of word is 賑やか, and why does it need です?

賑やか (にぎやか) is a な-adjective.

な-adjectives behave a bit like nouns grammatically:

Dictionary form: 賑やか
Polite predicate: 賑やかです
Plain predicate: 賑やかだ
Before a noun: 賑やかな公園 (“a lively park”)

You need だ/です to make it a predicate (to say “X is lively”):

公園は賑やかです。 – “The park is lively.” (polite)
公園は賑やかだ。 – “The park is lively.” (plain)

So in this sentence, 賑やかです is the final predicate, making the whole statement polite.

Who or what is 賑やか in this sentence? The park? The people?

The grammar points to the park (the situation at the park) being 賑やか.

The underlying structure is:

公園は、大勢の人がいて、賑やかです。

The topic 公園 (“the park”) is understood even if は is not written:

(公園は) 公園に大勢の人がいて賑やかです。

So the idea is:

“As for the park, there are many people there, and (the park) is lively.”

賑やか usually describes a place or atmosphere, not individual people:

賑やかな通り – a lively street
賑やかな教室 – a lively classroom

Can I say 公園で大勢の人がいて賑やかです instead? Is that wrong?

公園で大勢の人がいて賑やかです sounds unnatural.

Reasons:

With いる (“to be / exist”), the standard particle for the location of existence is に, not で.
で would make listeners expect an action (e.g. 遊ぶ, 話す, 歌う).

Natural patterns:

公園に大勢の人がいる – “There are many people in the park.”
公園で大勢の人が遊んでいて賑やかです – “A lot of people are playing in the park, so it’s lively.”

So keep に with いる here:
公園に大勢の人がいて賑やかです。

Why is the particle が used with 人? Could we use は instead: 大勢の人は?

In 大勢の人がいる, が marks the subject of existence:

(公園に) 大勢の人が いる
→ “There are many people (in the park).”

If you changed it to 大勢の人は, the nuance changes:

大勢の人はいて賑やかです
→ “As for the many people, they are there and (it) is lively.”

This feels odd because:

We normally talk about the place (the park) or situation being lively, not the people “as a topic”.
The existence construction 〜がいる / 〜がある very commonly uses が, not は.

Natural patterns:

公園に大勢の人がいる。 ✅
公園は大勢の人がいて賑やかです。 ✅ (topic = park)
✕ 公園に大勢の人はいる。 – possible in some contrastive contexts, but not as a neutral description.

Can I change the word order, like 大勢の人が公園にいて賑やかです? Is that okay?

Yes, 大勢の人が公園にいて賑やかです is grammatically correct and understandable.

Nuance:

公園に大勢の人がいて賑やかです。
→ Slightly more neutral, location first (“In the park, many people are there…”).
大勢の人が公園にいて賑やかです。
→ Puts a bit more emphasis on “many people” first (“Many people are in the park, and it’s lively.”).

Japanese word order is somewhat flexible as long as:

Particles are correct (に, が, etc.).
The predicate is at the end (賑やかです).

The original order is more typical for a location description, but your variant is fine.

Could I say 人が大勢いて賑やかです instead of 大勢の人がいて賑やかです? Is there a difference?

Both are natural, with a slight difference in feel.

大勢の人がいて賑やかです。
→ Literally “A large number of people are there and it’s lively.”
The “many-ness” is built into a phrase (大勢の人).
人が大勢いて賑やかです。
→ Literally “People are there in large numbers and it’s lively.”
This sounds a bit more like “people (as a category) are there in large numbers.”

Nuance is subtle; both mean essentially “There are many people, so it’s lively.” The original 大勢の人が… is perhaps a little more common in basic learner sentences.

Does 賑やか mean “noisy” in a bad way, or is it positive?

賑やか is usually positive or neutral, closer to:

“lively”
“bustling”
“full of activity”

Examples:

祭りで町はとても賑やかです。 – “The town is very lively during the festival.”
賑やかな通りが好きです。 – “I like lively streets.”

If you want to emphasize annoying noise, you’d more likely use:

うるさい – “noisy, loud, annoying”

So in this sentence, 賑やかです suggests a lively, pleasant atmosphere, not necessarily unpleasant noise.

Why is there です at the end instead of だ? What’s the difference?

です and だ both act like “is/are” for nouns and な-adjectives, but:

です – polite
だ – plain / casual

With 賑やか:

賑やかです。 – polite (to listener, like talking to a stranger, teacher, etc.)
賑やかだ。 – plain (diary, close friends, inner monologue, etc.)

So:

公園に大勢の人がいて賑やかです。 – what you’d typically say in a polite context (textbooks, talking to someone you’re not very close to).
公園に大勢の人がいて賑やかだ。 – fine in casual speech or writing.

Where is the topic は? Can I say 公園は大勢の人がいて賑やかです?

Good observation: the original sentence has no explicit は, but the topic is understood as “the park / the situation in the park”.

You can absolutely say:

公園は大勢の人がいて賑やかです。

This is very natural and arguably clearer for learners:

公園は – as for the park,
大勢の人がいて – there are many people (there),
賑やかです – (it) is lively.

In casual conversation, Japanese often omit obvious topics, especially if they have just been mentioned or are clear from context. That’s why just:

公園に大勢の人がいて賑やかです。

is perfectly normal in real speech.