gongwone beotkkochi pieoseo saramdeuri sajineul manhi jjikgo isseosseoyo.

This is a very common question, because both 에 and 에서 can relate to places.

In this sentence, 공원에 works naturally because the park is being presented as the location where the state/event exists or appears: cherry blossoms bloomed in the park.

A helpful way to think about it:

• 에 often marks a place someone/something is, goes to, or where something appears/happens in a descriptive way
• 에서 often marks the place where someone actively does an action

So in the sentence:

• 공원에 벚꽃이 피어서 = because cherry blossoms bloomed in the park
• 사람들이 사진을 많이 찍고 있었어요 = people were taking lots of photos

If you only look at the second clause, 공원에서 사람들이 사진을 많이 찍고 있었어요 would also be possible, because 찍다 is an action. But in the full sentence, 공원에 벚꽃이 피어서 is very natural because the first clause centers on 벚꽃이 피다.

이/가 marks the subject.

Here, 벚꽃이 피다 means the cherry blossoms bloom. So 벚꽃이 is the thing that is blooming.

This is straightforward subject marking:

• 벚꽃이 피다 = cherry blossoms bloom
• 사람들이 찍다 = people take photos

So the sentence has two different subjects in two different parts:

• 벚꽃이 for the blooming
• 사람들이 for the photo-taking

That is completely normal in Korean.

Here, -아서/어서 connects two clauses and usually gives a sense like:

• because
• so
• sometimes simply and then, depending on context

In this sentence, 피어서 is best understood as giving the reason/background for what follows:

• 벚꽃이 피어서 사람들이 사진을 많이 찍고 있었어요
• Because the cherry blossoms were blooming, people were taking lots of photos

So 피어서 is not just two separate actions stuck together. It shows that the blooming cherry blossoms are the reason or situation that led to people taking pictures.

Good question. Both forms connect clauses, but they do different jobs.

• -고 usually means simple connection: and
• -아서/어서 often adds cause, reason, or natural connection

Compare:

• 벚꽃이 피고 사람들이 사진을 찍었어요
  = The cherry blossoms bloomed, and people took photos.
  This sounds more like a simple sequence/listing.

• 벚꽃이 피어서 사람들이 사진을 찍었어요
  = Because the cherry blossoms bloomed, people took photos.
  This shows a clearer relationship.

In your sentence, the second part happens because of the first part, so 피어서 is the better choice.

Both are possible in some contexts, but they feel different.

• 이/가 marks the subject more neutrally
• 은/는 marks the topic and often adds contrast, emphasis, or a broader statement

Here, 사람들이 사진을 많이 찍고 있었어요 sounds like a neutral observation: people were taking lots of photos.

If you said 사람들은, it could sound more like:

• As for the people, they were taking lots of photos
• or it may suggest contrast with someone/something else

So 사람들이 fits well because the speaker is simply describing what they saw.

찍고 있었어요 is the past progressive form, meaning:

• were taking
• were in the middle of taking

Structure:

• 찍다 = to take (a photo)
• 찍고 있다 = to be taking
• 찍고 있었어요 = was/were taking

This emphasizes that the action was ongoing at that time.

Compare:

• 사진을 많이 찍었어요 = They took a lot of photos
  This just states the completed action.

• 사진을 많이 찍고 있었어요 = They were taking a lot of photos
  This paints the scene as something in progress.

In descriptive storytelling, -고 있었어요 is very common because it helps create a vivid scene.

많이 means a lot, many times, or in large quantity, depending on context.

Here it modifies 찍고 있었어요, so it means people were taking photos a lot / many times.

So:

• 사진을 많이 찍고 있었어요 = were taking lots of photos

It does not mean the photos themselves were large. It describes the amount/frequency of taking photos.

Because 사진 is the direct object of 찍다.

• 사진 = photo
• 찍다 = to take (a photo), to shoot

So:

• 사진을 찍다 = to take photos

The particle 을/를 marks what the action is being done to.

Yes, absolutely.

This sentence is made of two linked clauses:

1. 공원에 벚꽃이 피어서
2. 사람들이 사진을 많이 찍고 있었어요

Each clause has its own subject:

• 벚꽃이 = the cherry blossoms bloomed
• 사람들이 = people were taking photos

English learners sometimes expect only one subject per sentence, but Korean often links multiple clauses together, and each clause can have its own subject. That is completely normal and natural.

Korean nouns usually do not need a special plural form when the meaning is clear from context.

So 벚꽃 can mean:

• cherry blossom
• cherry blossoms

In this sentence, it is naturally understood as cherry blossoms in a general sense.

You could add 들 to some nouns for plural, but with things like flowers, people as a group, or general scenery, Korean often leaves the noun unmarked.

So 벚꽃이 피어서 naturally sounds like the cherry blossoms were blooming.

있었어요 is in the polite informal style, often called 해요체.

That means it is:

• polite
• natural in everyday conversation
• less formal than 있었습니다
• more polite than plain casual 있었어

So the full sentence sounds like a normal polite spoken description, such as something you might say in conversation, class, or a polite written caption.

Yes, Korean word order is flexible, though some orders sound more natural than others.

The original sentence:

• 공원에 벚꽃이 피어서 사람들이 사진을 많이 찍고 있었어요.

This is very natural because it presents:

1. the setting/background
2. the result or scene that followed

You could sometimes move pieces around for emphasis, for example:

• 벚꽃이 공원에 피어서 사람들이 사진을 많이 찍고 있었어요.

But this is less natural than the original, because 공원에 벚꽃이 피어서 flows more smoothly as a scene-setting phrase.

So yes, Korean allows movement, but the original order is a very natural descriptive order.

Yes. In casual pronunciation and in some writing styles, 피어서 may be contracted to 펴서.

So:

• 피어서
• 펴서

Both can appear, and both come from 피다.

However, 피어서 is very clear and standard for learners to recognize. If you later hear 벚꽃이 펴서, that is related and not a completely different verb.

Korean often leaves those kinds of linking words implicit because the grammar already shows the relationship.

In English, you might say:

• The cherry blossoms were blooming in the park, so people were taking lots of pictures there.

Korean does not need extra words like so or there if the meaning is clear:

• 피어서 already gives the because/so connection
• 공원에 already establishes the location

So Korean often sounds more compact than English, even when the meaning is just as clear.