da-eum dare eonni joreopsigi isseoseo gajogi modu hakgyoe ga.

에 marks a time point here, so 다음 달에 means next month or more literally in next month.

In Korean, 에 is often used with specific times:

• 월요일에 = on Monday
• 세 시에 = at 3 o’clock
• 다음 달에 = next month

It can be omitted in some cases with time expressions, but using 에 is very common and natural here.

Both mean older sister, but they depend on the speaker.

• 언니 = what a female speaker calls her older sister
• 누나 = what a male speaker calls his older sister

So this sentence suggests the speaker is female.
If a male speaker said the same thing, it would normally be:

• 다음 달에 누나 졸업식이 있어서 가족이 모두 학교에 가.

Korean often leaves out possessives like my, your, and our when the meaning is obvious from context.

So:

• 언니 졸업식 naturally means my older sister’s graduation ceremony here

Saying 내 언니 졸업식 is possible, but it is often unnecessary unless you want to emphasize my sister, not someone else’s.

This is very common with family terms in Korean.

졸업식 means graduation ceremony.

It is made of:

• 졸업 = graduation
• 식 = ceremony / formal event

So 졸업식 is literally something like graduation ceremony event.

있어서 comes from 있다 + -아서/어서.

• 있다 = to exist / to have
• -아서/어서 = because, so, and then, since

So 졸업식이 있어서 means:

• because there is a graduation ceremony
• since there is a graduation ceremony

In this sentence, it gives the reason for the second clause:

• 언니 졸업식이 있어서 가족이 모두 학교에 가.
• Because my older sister has a graduation ceremony, the whole family goes/is going to school.

In 졸업식이 있어서, 이/가 marks 졸업식 as the subject of 있다.

With 있다 meaning there is/exists, the thing that exists is usually marked by 이/가:

• 행사가 있어요. = There is an event.
• 시험이 있어요. = There is an exam.
• 졸업식이 있어요. = There is a graduation ceremony.

So 졸업식이 있어서 literally means because a graduation ceremony exists.

Both are possible, but they feel a little different.

• 가족이 모두 학교에 가. = a neutral statement about who is going
• 가족은 모두 학교에 가. = puts more topic focus on the family, often with a contrastive feeling

Here, 가족이 모두 simply presents the whole family as the subject of the action.

Also, 모두 means all or everyone, so:

• 가족이 모두 = the family all / the whole family

모두 means all or everyone.

In this sentence, it emphasizes that every family member is going:

• 가족이 모두 학교에 가. = The whole family goes to school.

It can appear in slightly different positions too:

• 가족 모두 학교에 가.
• 모든 가족이 학교에 가. (less natural for this exact meaning; sounds more like all families unless context is clear)

So 가족이 모두 is a natural way to say the whole family.

In Korean, the present tense is often used for future events when the time is already clear from context.

Because the sentence begins with 다음 달에 (next month), Korean does not need a special future form here.

So:

• 다음 달에 ... 가. = Next month ... is going / will go

This is very natural in Korean.

You could also use a future form, but it is not necessary:

• 갈 거야 = will go

The plain present often sounds simple and natural for scheduled or expected future actions.

The phrase 학교에 가다 can mean either:

• go to school physically
• attend school in a more general sense

Context tells you which meaning is intended.

Here, because there is a graduation ceremony, 학교에 가 clearly means:

• go to the school
• go to school for the ceremony

So this is about physically going to the location, not about being a student in general.

With movement verbs like 가다 (to go), 에 marks the destination.

• 학교에 가다 = to go to school
• 집에 가다 = to go home
• 회사에 가다 = to go to the office

So 학교에 means to the school.

It is casual/plain style because it ends in 가 instead of 가요 or 갑니다.

• 가 = casual
• 가요 = polite
• 갑니다 = formal

So the sentence sounds like something you might say to a friend or someone close.

Polite version:

• 다음 달에 언니 졸업식이 있어서 가족이 모두 학교에 가요.

More formal version:

• 다음 달에 언니 졸업식이 있어서 가족이 모두 학교에 갑니다.

Yes, -아서/어서 can sometimes connect events in a looser and then/so way, depending on context. But in this sentence, the most natural reading is clearly because/since.

So here:

• 언니 졸업식이 있어서 가족이 모두 학교에 가. is best understood as:
• Because my older sister has a graduation ceremony, the whole family is going to school.

The first clause explains the reason for the second clause.

You would normally change 언니 to 누나 and use a polite ending:

• 다음 달에 누나 졸업식이 있어서 가족이 모두 학교에 가요.

That means the same thing, but it matches a male speaker talking about his older sister and uses polite speech.