chinguga oneul gongwone sarami manhdaeyo.

-대요 shows reported information. The speaker is not just saying there are many people as a direct observation; they are passing along what someone else said.

In this sentence, the source is 친구. So the nuance is like:

• My friend says there are a lot of people at the park today.
• I hear from a friend that there are a lot of people at the park today.

The final -요 makes the sentence polite.

Yes. 많대요 is a contracted form of 많다고 해요.

So this sentence can be understood as:

친구가 오늘 공원에 사람이 많다고 해요.
→ 친구가 오늘 공원에 사람이 많대요.

Both mean the same thing. The shorter form is very common in everyday speech.

Because they belong to two different parts of the sentence.

• 친구가 marks friend as the subject of the reporting part: ...말해요 / ...해요
• 사람이 marks people as the subject of 많다

A helpful way to see it is this fuller structure:

친구가 [오늘 공원에 사람이 많다고] 해요.

So:

• 친구가 = my friend is the one saying it
• 사람이 = people are the ones being described as many

This is very normal in Korean, especially with reported speech.

No. 친구 is not the subject of 많다.

The thing that is many is 사람:

• 사람이 많다 = there are many people

친구가 only tells you where the information came from: the friend said it.

Because 에 is commonly used for the place where someone or something exists or is located.

Here, 사람이 많다 describes a situation at the park, not an action being performed there. So 공원에 사람이 많아요 is the natural pattern.

Compare:

• 공원에 사람이 많아요 = There are many people at the park
• 공원에서 운동해요 = I exercise at the park

에서 is usually used for the place where an action happens.
에 is usually used for location/existence.

This is a very common Korean pattern.

Literally, 사람이 많다 is closer to people are many, but in natural English it means there are many people.

Korean often uses Noun + 이/가 많다 to express there are a lot of Nouns.

For example:

• 학교에 학생이 많아요 = There are many students at school
• 길에 차가 많아요 = There are many cars on the road

So 공원에 사람이 많아요 is a standard way to say there are many people in the park.

Yes, the word order can change.

Korean word order is more flexible than English, as long as the particles stay attached to the right words. So these are all possible:

• 친구가 오늘 공원에 사람이 많대요.
• 오늘 친구가 공원에 사람이 많대요.
• 친구가 공원에 오늘 사람이 많대요.

The basic meaning stays very similar. The difference is mostly emphasis or flow.

The original sentence sounds natural and neutral.

친구가 is the neutral choice here. It simply marks friend as the source of the information.

If you say 친구는, then friend becomes the topic, often with a contrastive feeling. It can sound like:

• As for my friend, they say there are many people at the park today

That is possible, but it gives a slightly different nuance. In a plain statement, 친구가 is more natural.

Usually no.

If you personally saw that the park was crowded, you would normally say:

• 오늘 공원에 사람이 많아요. = There are a lot of people at the park today.

If you are reacting to what you see right now, you might also say:

• 오늘 공원에 사람이 많네요.

Using -대요 suggests that you are reporting what someone else said, not just stating your own direct observation.

Because Korean often leaves out possessives like my, your, or his/her when they are obvious from context.

So 친구 can often be translated as my friend in English, even though Korean just says friend.

Also, Korean does not have articles like a and the, so depending on context, 친구 could mean:

• my friend
• a friend
• the friend

English usually has to choose one, but Korean often leaves that to context.

The speaker is not stated, because Korean often omits I when it is obvious from context.

So the full idea is something like:

• I’m telling you that my friend says there are many people at the park today.

But Korean does not need to say I here. Omitting it sounds completely natural.