chingudeuri seupoilleoreul haeseo ajik an bon deurama gyeolmareul imi ara beoryeosseo.

-이/가 marks 친구들 as the grammatical subject of the clause: the friends are the ones who did the spoiling.

Using 친구들은 would be possible in some contexts, but it would feel more like setting up friends as a topic, often with some contrast or background nuance. In this sentence, 친구들이 is the more straightforward choice:

• 친구들이 스포일러를 해서... = My friends spoiled it, so...

So here, -이/가 simply identifies who caused the situation.

Here, 스포일러 is a loanword from English spoiler. In Korean, people often use it as a noun and combine it with 하다:

• 스포일러를 하다 = to give a spoiler / to spoil something

So:

• 스포일러를 해서 = because they spoiled it / by giving spoilers

The -아서/어서 form here connects this clause to the next one and often gives a cause/reason meaning.

So the structure is:

• 친구들이 스포일러를 하다 → 친구들이 스포일러를 해서
• because my friends gave spoilers...

This is a noun-modifying clause, which is very common in Korean.

Break it down:

• 보다 = to watch
• 본 = watched / that someone watched
  (the noun-modifying form of 보다)
• 안 본 = not watched
• 아직 안 본 = not watched yet

Then this whole phrase modifies 드라마:

• 아직 안 본 드라마 = a drama that I haven’t watched yet

In English, we often need words like that or a relative clause. Korean just puts the modifying verb phrase directly before the noun.

The subject is omitted, but it is understood from context as I.

So:

• 아직 안 본 드라마 literally = the drama [I] haven’t watched yet

Korean very often leaves out subjects and objects when they are obvious from context. Since the speaker later says 알아 버렸어, it is natural to understand that the speaker is the one who had not watched the drama yet.

Both are possible.

• 드라마 결말 = drama ending
• 드라마의 결말 = the ending of the drama

Korean often leaves out 의 in everyday speech, especially when two nouns are closely connected and the meaning is clear.

So:

• 아직 안 본 드라마 결말 sounds natural and conversational
• 아직 안 본 드라마의 결말 is also correct, but slightly more explicit

This kind of noun-noun combination is very common in Korean.

They are not contradictory because they refer to different things.

• 아직 안 본 = haven’t watched yet
• 이미 알아 버렸어 = already ended up finding out

So the meaning is:

• I still haven’t watched the drama,
• but I already know the ending.

This contrast is actually very natural:

• 아직 goes with the fact that watching has not happened
• 이미 goes with the fact that knowing the ending has already happened

알아 버렸어 uses the auxiliary verb 버리다, which adds a nuance of:

• completion
• something happening fully
• often regret, annoyance, or an unwanted result

So:

• 알았어 = I found out / I know
• 알아 버렸어 = I ended up finding out, unfortunately / I found out and now it’s done

In this sentence, 버리다 strongly suggests that learning the ending was not what the speaker wanted. That fits the spoiler situation very well.

This is because 알다 conjugates irregularly in actual usage:

• dictionary form: 알다
• informal present/past stem used before endings: 알아-

So:

• 알다
  • -어 → 알아
• 알아 + 버렸어 → 알아 버렸어

This is just the normal conjugated form you have to learn as part of how 알다 behaves in sentences.

Yes. -어/아 style is casual, informal speech.

So 알아 버렸어 is something you would say to:

• a friend
• someone younger
• someone you are close to

A more polite version would be:

• 친구들이 스포일러를 해서 아직 안 본 드라마 결말을 이미 알아 버렸어요.

The grammar and nuance stay the same; only the speech level changes.

Yes, it is understandable and natural, but in everyday speech many Koreans also say:

• 스포 = spoiler / spoil
• 스포하다 = to spoil
• 스포 당하다 = to get spoiled

So similar natural sentences might be:

• 친구들이 스포해서...
• 친구들한테 스포 당해서...

The sentence you were given is completely fine, but learners should know that 스포 is very common in real conversation.