eoje yageunhaeseo oneureun jeongmal pigonhae.

야근하다 means to work overtime.

So:

• 야근 = overtime, working late
• 야근하다 = to do overtime work / work overtime
• 야근해서 = because (someone) worked overtime / having worked overtime

In this sentence, -해서 connects the first clause to the second and gives a reason/cause:

• 어제 야근해서 = because I worked overtime yesterday
• 오늘은 정말 피곤해 = today I’m really tired

So 야근해서 is functioning like because I worked overtime.

-아서/어서/해서 is a very common way to link two clauses when the first one is the reason for the second.

Here it sounds natural and conversational:

• 어제 야근해서 오늘은 정말 피곤해. = I worked overtime yesterday, so I’m really tired today.

Compared with other forms:

• -해서: neutral, very common, natural in everyday speech
• -니까: often feels a bit more explicit or assertive, like since/because
• -니: more written, formal, or old-fashioned in many situations

So 해서 is probably the most natural everyday choice here.

The particle 은 marks 오늘 as the topic and often adds a sense of contrast.

So:

• 오늘 정말 피곤해 = Today I’m really tired
• 오늘은 정말 피곤해 = As for today, I’m really tired / I’m really tired today (especially, in contrast to something else)

In this sentence, the contrast is easy to feel:

• 어제 = yesterday, I worked overtime
• 오늘은 = today, I’m tired

So 은 helps highlight today as the relevant topic/result of what happened yesterday.

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

So even though the sentence does not say I, the implied subject is usually:

• I worked overtime yesterday, so I’m really tired today

If needed, you could add it:

• 저는 어제 야근해서 오늘은 정말 피곤해요.
• 나는 어제 야근해서 오늘은 정말 피곤해.

But in normal conversation, leaving it out sounds very natural.

피곤하다 is the dictionary form, meaning to be tired.

In actual speech, it gets conjugated:

• 피곤하다 = dictionary form
• 피곤해 = casual informal speech
• 피곤해요 = polite speech
• 피곤합니다 = formal speech

So in this sentence:

• 피곤해 sounds casual, like talking to a friend or to yourself

If you wanted to make it polite, you would say:

• 어제 야근해서 오늘은 정말 피곤해요.

Yes, 정말 means really or truly, and here it modifies 피곤해:

• 정말 피곤해 = really tired

Other common options:

• 너무 피곤해 = so tired
• 아주 피곤해 = very tired
• 진짜 피곤해 = really tired (more casual/conversational)

Nuance-wise:

• 정말 is very common and neutral
• 진짜 sounds more casual
• 너무 is extremely common in speech, but depending on context it can feel like so/too

This is a very common learner question.

Even though the overtime happened in the past, Korean often does not mark past tense separately in this kind of -아서/어서/해서 connection if the time is already clear from context.

Here, 어제 already tells you the action happened yesterday:

• 어제 야근해서 = because I worked overtime yesterday

So the sentence is completely natural as written.

You may sometimes hear forms like 했어서, but for standard, simple cause-and-effect sentences like this, 해서 is the normal and natural choice.

A more literal breakdown would be:

• 어제 = yesterday
• 야근해서 = because worked overtime
• 오늘은 = as for today
• 정말 = really
• 피곤해 = am tired

So very literally:

• Yesterday, because I worked overtime, today I’m really tired.

Natural English would usually be:

• I worked overtime yesterday, so I’m really tired today.
• Because I worked overtime yesterday, I’m really tired today.

This is a good example of how Korean often puts the reason first and the main result at the end.

Yes, technically it can, because the subject is omitted.

Depending on context, it could mean:

• I worked overtime yesterday, so I’m really tired today
• He worked overtime yesterday, so he’s really tired today
• She worked overtime yesterday, so she’s really tired today

But if someone says this about themselves in casual conversation, the default interpretation is usually I.

If you want to make the subject clear, you can add it:

• 민수는 어제 야근해서 오늘은 정말 피곤해. = Minsu worked overtime yesterday, so he’s really tired today.

Usually, 야근하다 strongly suggests working late/overtime, especially in a job context such as office work or company work.

It is commonly used for employees staying late after normal work hours.

If you just want to say stayed up late in a general sense, Korean would often use something else, such as:

• 늦게 자다 = to go to bed late
• 밤새다 = to stay up all night

So 야근하다 is specifically about work, not just being awake late.

Yes, that is grammatically fine and sounds natural.

• 어제 야근해서 오늘 정말 피곤해
• 어제 야근해서 오늘은 정말 피곤해

Both work.

The version with 은 gives a little more topic/contrast feeling, as if emphasizing today as the result:

• Yesterday I worked overtime, and today, I’m really tired

Without 은, it sounds a bit more straightforward and slightly less contrastive.

Both are very common in speech.