oneureun eopmuga manhaseo yageunhaeyo.

은 is the topic particle. In 오늘은, it sets today as the topic, often with a nuance like as for today or today in particular.

So:

• 오늘은 = as for today / today
• 오늘이 would make today the grammatical subject, which sounds different and is less natural in this sentence
• 오늘 with no particle is possible in casual speech, but 오늘은 sounds more complete and natural here

The sentence has a slight contrastive feeling too: Today, I’m working late because there’s a lot of work.

In 업무가 많아서, 가 marks 업무 as the subject of 많다.

• 업무 = duties, tasks, work-related business
• 많다 = to be many / to be a lot

So 업무가 많다 literally means the tasks/work are many or more naturally there is a lot of work.

Korean often uses a subject marker with descriptive verbs like 많다. In English we say I have a lot of work, but Korean usually structures it more like work is 많다.

Both can relate to work, but they are not exactly the same.

• 일 is the general everyday word for work, job, or even thing to do
• 업무 is more formal and specifically means job duties, tasks, or official work

So:

• 일이 많아요 = more everyday, common speech
• 업무가 많아요 = more formal or workplace-style wording

In an office or business context, 업무 sounds very natural.

많아서 comes from 많다 + -아서/어서, which connects two clauses and often means because or so.

• 많다 = to be many / to be a lot
• 많아서 = because there is a lot / since there is a lot

So:

• 업무가 많아서 야근해요 = Because there is a lot of work, I work late / I’m working overtime

This -아서/어서 form can connect actions or states, and here it gives the reason for the second part.

야근 is a noun meaning night work or overtime work, and 하다 turns it into a verb:

• 야근 = overtime / working late
• 야근하다 = to work overtime

So 야근해요 is the present polite form of 야근하다.

This noun + 하다 pattern is extremely common in Korean. For example:

• 공부하다 = to study
• 운동하다 = to exercise
• 야근하다 = to work overtime

It can mean either, depending on context.

Korean present tense often covers both:

• I work overtime
• I am working overtime
• sometimes even I will work overtime if the context is about a near-future plan

In this sentence, because it starts with 오늘은 and gives a reason, the most natural English meaning is:

• I’m working overtime today because I have a lot of work.

But grammatically, Korean is not making the same strict present-progressive distinction that English does.

Korean often omits the subject when it is obvious from context.

In 오늘은 업무가 많아서 야근해요, there is no explicit 저는 or 제가, but the listener naturally understands that the speaker is talking about themselves.

Korean does this very often. If the subject is clear, it is usually left out.

A fuller version could be:

• 저는 오늘은 업무가 많아서 야근해요.

But this sounds a bit repetitive or unnecessary in many situations.

야근해요 is in the 해요-style, which is polite and very common in everyday conversation.

Levels for comparison:

• 야근해요 = polite, standard conversational style
• 야근합니다 = more formal, often used in announcements, presentations, or very formal workplace speech
• 야근해 = casual, used with close friends or someone younger

So this sentence sounds polite and natural in normal conversation.

Yes, you could. It would still be understandable and natural in many situations.

But 오늘은 adds a helpful topic or contrast nuance:

• 오늘 업무가 많아서 야근해요 = Today I’m working overtime because there’s a lot of work.
• 오늘은 업무가 많아서 야근해요 = As for today, I’m working overtime because there’s a lot of work.

The version with 은 sounds a little more like the speaker is setting today apart from other days.

Not always. -아서/어서 can connect ideas in more than one way.

It can mean:

• because
• and then / so
• a natural sequence of events

In this sentence, the reason meaning is the natural one:

• 업무가 많아서 야근해요 = Because there is a lot of work, I work overtime

But in other sentences, it may simply link events rather than give a reason.

Yes, very natural.

It sounds like something you might say to a coworker, friend, or manager depending on the situation. Because it uses 업무 and 야근, it has a clear workplace feel.

It is especially natural if you want to explain why you cannot leave on time or why you are busy today.