naneun hubaerang doseogwaneseo gati gongbuhaneunde, geu hubaega jeongong sueobi eoryeopdago haeseo naega memoreul boyeo jwosseo.

후배 means a junior in a school, workplace, or other social hierarchy—someone who entered later than you or has less seniority.

It does not simply mean younger person. A 후배 could even be older than you in age, but still be your junior in that group.

The opposite is 선배, meaning senior.

-랑 means with in casual speech, so 후배랑 means with my junior.

Other similar forms are:

• 후배하고
• 후배와 / 후배과 (more formal/written, though 와/과 depends on pronunciation)

So:

• 후배랑 도서관에서 같이 공부하는데
  = I was studying together with a junior at the library...

Because -에서 marks the place where an action happens.

• 도서관에서 공부하다 = study at the library

By contrast, -에 is often used for:

• destination: 도서관에 가다 = go to the library
• existence: 도서관에 있다 = be at/in the library

Since studying is an action taking place there, -에서 is correct.

같이 means together.

So 후배랑 같이 공부하다 means to study together with a junior.

In many situations, Korean can omit 같이 if -랑 already makes the meaning clear, but keeping it sounds natural and emphasizes doing the activity together.

Here, -는데 connects the clause to what comes next and gives background context.

So 나는 후배랑 도서관에서 같이 공부하는데 is like:

• I was studying with a junior at the library, and...
• While I was studying with a junior at the library...
• I was studying with a junior at the library, when...

It does not necessarily mean a strong but here.
Instead, it sets the scene for the next event.

Korean often repeats nouns where English would use he, she, or just omit them.

So 그 후배가 helps clearly identify the subject of the next clause:

• 그 후배가 전공 수업이 어렵다고 해서...

This is especially useful because the sentence is switching attention from me to the junior, and then back to me later.

Also, Korean uses pronouns less often than English, and repeating the noun can sound more natural than using 그가 or 걔가 in many contexts.

전공 means major or field of study, and 수업 means class or course.

So 전공 수업 means:

• a class in one’s major
• a major course

In this sentence, it refers to the junior’s class related to their academic major.

This is a very common pattern.

Break it down:

• 어렵다 = to be difficult
• 어렵다고 = that it is difficult (quoted/reported content)
• -아/어서 → here as 해서 = because / since / and so

So 어렵다고 해서 means:

• because he said it was difficult
• since he said it was difficult

The important nuance is that the reason comes from what the junior said, not just from the speaker’s own description.

Because 어려워서 would mean:

• because the major class was difficult

That presents the difficulty more directly as a fact from the speaker’s point of view.

But 어렵다고 해서 means:

• because the junior said the class was difficult

So the sentence is not only telling you the reason; it is also showing that the reason came through the junior’s words. That reported-speech nuance is important.

This is about topic versus subject focus.

• 나는 sets up me as the overall topic:
  As for me...
• 내가 later highlights I as the one who did the action:
  I showed my notes

In the middle, the subject changes:

• 그 후배가 = the junior is the subject of said it was difficult
• then 내가 = I am the subject of showed the notes

So 내가 helps clearly mark the shift back to the speaker and can add a slight sense of I was the one who...

보여 주다 literally means to show (something) to someone, and it often carries a helpful or benefactive nuance.

So 메모를 보여 줬어 means:

• I showed my notes
• more literally, I showed the notes to him/her

In Korean, the indirect object is often omitted if it is obvious from context. Here, the notes are being shown to the junior.

Also, 보여 줬어 is the past casual form of 보여 주다.

In Korean, -아/어 주다 often adds the idea of doing something for someone.

So:

• 보이다 / 보여 주다 can both relate to showing,
• but 보여 주다 strongly suggests showing it to someone.

In this sentence, that fits perfectly because the speaker is helping the junior by showing their notes.

This is very normal in Korean.

Only the final main action is explicitly in past tense:

• 보여 줬어 = showed

Earlier parts like:

• 공부하는데
• 어렵다고 해서

often stay in a non-past or neutral connective form because they function as background information or linked clauses, not the main final event.

This is similar to how English sometimes says:

• I’m at the library studying, and then...
• even when telling a past story

So the tense system here is natural and conversational.

No. That is a very common learner misunderstanding.

-는데 can mean different things depending on context, such as:

• giving background
• soft contrast
• setting the scene
• leading into a result
• introducing a situation before asking/requesting something

In this sentence, it is mostly background/scene-setting, not a strong but.

So translating it as but here would sound too forceful.

Yes. The sentence is in casual, informal speech.

The final ending -어 / -았어 / -었어 is used with friends, younger people, or in informal conversation:

• 줬어 = casual past
• more polite version: 보여 줬어요
• more formal/written: 보여 주었습니다

Because the sentence ends casually, the whole sentence sounds like relaxed spoken Korean.