oneureun doseogwaneseo haru jongil gongbuhaesseoyo.

The particle 은 marks 오늘 as the topic of the sentence.

• 오늘 = today
• 오늘은 = as for today / today, ...

This often gives a slight sense of contrast or sets the scene for what follows. In natural English, it may not always need to be translated directly.

So:

• 오늘 공부했어요. = I studied today.
• 오늘은 공부했어요. = As for today, I studied. / Today, I studied.

In this sentence, 오늘은 helps frame the whole statement around today.

Because 에서 marks the place where an action happens.

• 도서관에서 공부했어요. = I studied at the library.

By contrast, 에 is often used for:

• destination: 도서관에 갔어요 = I went to the library
• time: 3시에 만나요 = Let's meet at 3

A very common beginner distinction is:

• 에 가다 = go to
• 에서 하다 = do something at

So:

• 도서관에 갔어요. = I went to the library.
• 도서관에서 공부했어요. = I studied at the library.

하루 종일 means all day long.

Breakdown:

• 하루 = one day / a day
• 종일 = throughout / all the way through

Together, 하루 종일 means the entire day from beginning to end.

Examples:

• 하루 종일 잤어요. = I slept all day.
• 하루 종일 비가 왔어요. = It rained all day.

In your sentence, it tells us how long the studying lasted.

In Korean, time expressions and adverb-like phrases usually come before the verb.

Korean word order is often:

time + place + duration/manner + verb

So in this sentence:

• 오늘은 = today
• 도서관에서 = at the library
• 하루 종일 = all day long
• 공부했어요 = studied

Since the verb normally comes at the end in Korean, everything describing the action tends to appear before it.

공부했어요 is the past polite form of 공부하다 (to study).

Breakdown:

• 공부 = study
• 하다 = to do
• 공부하다 = to study
• 공부했어요 = studied

How it changes:

• 공부하다
• 공부해요 = study / am studying
• 공부했어요 = studied

The 했어요 part comes from the past form of 하다.

Korean often leaves out the subject when it is obvious from context.

So 오늘은 도서관에서 하루 종일 공부했어요 naturally means something like:

• I studied at the library all day today
• or Today, I studied at the library all day

In real Korean, saying 저는 or 제가 every time would often sound repetitive if the speaker is already understood.

You could say:

• 저는 오늘은 도서관에서 하루 종일 공부했어요.

But in many situations, that is unnecessary.

공부했어요 is in the polite informal style, often called the -요 form.

It is very common in everyday speech when speaking politely to:

• strangers
• coworkers
• teachers
• older people
• people you do not know well

Compare:

• 공부했어요 = polite
• 공부했어 = casual/informal
• 공부했습니다 = more formal

So this sentence sounds natural and polite for normal conversation.

Yes, it is very natural.

It sounds like a normal statement someone might say in conversation, for example when talking about their day.

The sentence flows well because it follows a common Korean order:

• topic/time: 오늘은
• place: 도서관에서
• duration: 하루 종일
• action: 공부했어요

This is a very standard and useful sentence pattern.

Yes, Korean word order is somewhat flexible, as long as the verb stays near the end and the particles make the roles clear.

For example, these can also be understandable:

• 오늘은 하루 종일 도서관에서 공부했어요.
• 도서관에서 오늘은 하루 종일 공부했어요.

However, some orders sound more natural depending on what you want to emphasize. The original sentence is smooth and neutral.

Korean often changes word order for emphasis, but not completely freely. So beginners should first get comfortable with the most common pattern: time + place + duration + verb.

It can, but it does not have to.

The topic particle 은 often carries a subtle contrastive feeling, such as:

• today as opposed to other days
• as for today...

Depending on context, it might suggest something like:

• Today, I studied all day at the library (maybe unlike yesterday)

But sometimes it is just a natural way to introduce the topic, with no strong contrast intended.

So the contrast is often soft and depends on situation and tone.

Yes, you can.

• 오늘 도서관에서 하루 종일 공부했어요.
• 오늘은 도서관에서 하루 종일 공부했어요.

Both are grammatical and natural.

The difference is that 오늘은 marks today more clearly as the topic, while 오늘 without a particle can sound a bit more neutral or straightforward.

In everyday speech, both are possible. Using 은 is especially common when setting the scene or when there is a slight contrast.

Because English and Korean do not always match one-to-one in prepositions.

The particle 에서 means the location where an action takes place, but English may translate that as:

• at
• in
• from

depending on the situation.

So:

• 도서관에서 공부했어요 is most naturally I studied at the library
• But I studied in the library could also work in English depending on context

The important Korean point is not the exact English preposition, but that 에서 marks the place where the action happened.

Yes. 하루 종일 is a very useful expression and works with many actions or states.

Examples:

• 하루 종일 일했어요. = I worked all day.
• 하루 종일 기다렸어요. = I waited all day.
• 하루 종일 게임했어요. = I played games all day.
• 하루 종일 집에 있었어요. = I stayed home all day.

So it is a flexible phrase that simply means throughout the whole day.