jega sseun munjangeul seonsaengnimi gochyeo jusyeosseoyo.

It has two main parts:

• 제가 쓴 문장을 = the sentence that I wrote
• 선생님이 고쳐 주셨어요 = the teacher corrected it for me

So the full structure is:

• [제가 쓴] modifies 문장
• [제가 쓴 문장을] is the object
• 선생님이 is the subject of the main verb
• 고쳐 주셨어요 is the main verb phrase

A very literal breakdown is:

• 제가 = I
• 쓴 = wrote / written
• 문장을 = sentence + object marker
• 선생님이 = teacher + subject marker
• 고쳐 주셨어요 = corrected for me

Korean often puts the clause that describes a noun directly in front of that noun.

쓴 is the noun-modifying form of 쓰다, which means to write.

In this sentence, 쓴 means written or that I wrote, depending on how you translate it naturally.

So:

• 쓰다 = to write
• 쓴 문장 = the sentence that was written / the sentence I wrote

This is an example of a Korean relative clause. Instead of saying the sentence that I wrote with a separate word like that, Korean uses a verb form directly before the noun:

• 제가 쓴 문장 = the sentence I wrote

Also, this is why you do not say 제가 썼 문장.
썼어요 is a finite past-tense verb form used at the end of a clause, but 쓴 is the special modifier form used before a noun.

제가 is the polite or humble form of I used with the subject marker -가.

Breakdown:

• 저 = I, in a polite/humble way
• 제 = my / I + particle combination form based on 저
• 가 = subject marker

So 제가 means I as the subject, in polite speech.

Why not the others?

• 내가 is less polite and more casual.
• 저는 uses the topic marker -는, not the subject marker -가.

In 제가 쓴 문장, the speaker is identifying which sentence it is: the sentence that I wrote. Using -가 is very natural here.

You could sometimes say 제가 쓴 문장은... if the noun phrase were the topic of the sentence, but in your sentence it is the object, so 문장을 comes later.

Because 문장 is the object of the main verb 고쳐 주셨어요.

The teacher corrected what?
The sentence I wrote.

So:

• 문장 = sentence
• 문장을 = sentence + object marker

The whole object is actually:

• 제가 쓴 문장을 = the sentence that I wrote
  • object marker

The object marker attaches to the noun being corrected, which is 문장.

Because there are really two clauses here:

1. 제가 쓴 = I wrote
2. 선생님이 고쳐 주셨어요 = the teacher corrected it for me

Each clause has its own subject:

• 제가 is the subject of 쓴
• 선생님이 is the subject of 고쳐 주셨어요

So this does not mean Korean has two main subjects in one simple clause. It means one clause is modifying a noun, and the main clause has its own subject.

This is very common in Korean.

There is no separate word for that or which here.

Korean usually makes relative clauses by putting the descriptive clause directly before the noun:

• 제가 쓴 문장 = the sentence that I wrote
• literally, something like I-wrote sentence

So Korean does not need a separate relative pronoun in this kind of structure.

This is one of the biggest differences from English:

• English: the sentence that I wrote
• Korean: I wrote sentence

Of course, the natural English translation still needs that or sometimes leaves it out:

• the sentence I wrote
• the sentence that I wrote

It is made of several parts:

• 고치다 = to fix, correct
• -아/어 주다 = to do something for someone
• -시- = honorific marker
• -었- = past tense
• -어요 = polite ending

So:

• 고쳐 주셨어요 = corrected it for me with politeness and respect

A more detailed breakdown:

• 고치다 becomes 고쳐
• 주다 adds the meaning do it for someone
• 주셨어요 shows both past tense and respect toward the teacher

So the sentence is not just saying the teacher corrected it. It also implies the teacher did that as a helpful action for the speaker.

Using 주다 adds the nuance that the action was done for the speaker's benefit.

Compare:

• 선생님이 문장을 고치셨어요 = The teacher corrected the sentence.
• 선생님이 문장을 고쳐 주셨어요 = The teacher corrected the sentence for me.

The version with 주다 sounds more natural when someone helps you by correcting your work.

This is very common in Korean. Speakers often use -아/어 주다 when someone does something that benefits them.

Because the subject is 선생님 and teachers are normally treated with respect in Korean.

The honorific marker is -시-, which appears here inside 주셨어요.

So:

• 주셨어요 shows respect toward 선생님
• -어요 also makes the sentence polite overall
• 제 instead of 내 also fits polite speech

Without the honorific marker, the sentence would sound less respectful:

• 고쳐 줬어요 = corrected it for me

That would usually be inappropriate if you are referring to a teacher in normal polite conversation.

Yes, that is also correct.

Because Korean uses particles like -이/가 and -을/를, word order is more flexible than in English.

Both of these are natural:

• 제가 쓴 문장을 선생님이 고쳐 주셨어요
• 선생님이 제가 쓴 문장을 고쳐 주셨어요

The difference is mostly about emphasis or flow.

• 제가 쓴 문장을... puts the focus first on the sentence I wrote
• 선생님이... puts the teacher earlier in the sentence

In many contexts, the second version may sound a bit more neutral to an English speaker, but the original sentence is perfectly natural Korean.

It could definitely be something else, depending on what was corrected.

For example:

• 제가 쓴 글을 선생님이 고쳐 주셨어요.
  The teacher corrected the writing/text I wrote.

• 제가 쓴 숙제를 선생님이 고쳐 주셨어요.
  The teacher corrected the homework I did.

• 제가 쓴 답을 선생님이 고쳐 주셨어요.
  The teacher corrected the answer I wrote.

So the pattern to remember is:

• 제가 쓴 + noun
• 선생님이 + object + 고쳐 주셨어요

That pattern is very useful and productive in Korean.