sueobi kkeutnago naseo pianoreul yeonseuphaeyo.

Korean often leaves out the subject when it is already clear from context. So 수업이 끝나고 나서 피아노를 연습해요 can naturally mean:

• I practice the piano after class ends
• I’ll practice the piano after class ends

If you want to make the subject explicit, you can say:

• 저는 수업이 끝나고 나서 피아노를 연습해요.

That sounds fine too, but Korean usually prefers omission when possible.

Because 끝나다 means to end / to be over, and it is an intransitive verb. That means the thing that ends is marked as the subject, not the object.

So:

• 수업이 끝나다 = the class ends
• not 수업을 끝나다

If you want to say to end/finish the class as an action done by someone, you would use 끝내다, which is transitive:

• 수업을 끝내고 나서... = after finishing class...

So in your sentence, the class itself is ending, which is why 이 is used.

끝나고 나서 means after ending, after it ends, or once it’s over.

It comes from:

• 끝나다 = to end
• -고 나서 = after doing something

So:

• 수업이 끝나고 나서 = after class ends

This grammar emphasizes that one action happens first, and then the next action happens afterward.

It may look redundant at first, but -고 나서 is a very common fixed grammar pattern.

Compare:

• 끝나고 = and then it ends / after ending
• 끝나고 나서 = after it has ended

Using -고 나서 makes the sequence more explicit. It strongly suggests:

1. class ends first,
2. then piano practice happens.

So it is not just two separate words randomly put together; -고 나서 functions as one grammar pattern meaning after doing.

Grammatically, you might hear something like that, but 끝나고 나서 is clearer and more natural here.

That is because -아서/어서 can also express cause/reason, not just sequence. So:

• 수업이 끝나서... can sound like because class ended...

But your sentence is mainly about time order, not cause. So 수업이 끝나고 나서 is a better choice for saying after class ends.

Because 피아노 is the object of 연습하다.

• 연습하다 = to practice
• You practice something, so that thing takes 을/를

So:

• 피아노를 연습해요 = I practice the piano
• 한국어를 연습해요 = I practice Korean
• 발음을 연습해요 = I practice pronunciation

Even though English sometimes says just practice piano without an article, Korean still marks 피아노 as the object with 를.

Yes. 연습해요 comes from 연습하다.

Breakdown:

• 연습 = practice
• 하다 = to do

Together:

• 연습하다 = to practice

In the polite -아요/어요 style, 하다 becomes 해요, so:

• 연습하다 → 연습해요

This is the standard polite everyday form.

It can mean different things depending on context.

In Korean, the present tense often covers:

• habitual action: I practice
• current/future plan: I will practice
• sometimes ongoing action, depending on the situation

In this sentence, the most natural interpretations are:

• I practice the piano after class
• I’ll practice the piano after class

If the speaker is talking about a routine, it sounds habitual. If they are talking about today’s plan, it sounds future.

Korean often puts the time/background information first and the main action later.

So the structure is:

• After class ends
  • I practice the piano

This is very natural in Korean. In many cases, Korean sentences are organized like this:

1. background or setting
2. main action at the end

That is also why the final verb, 연습해요, is so important: the main action usually comes last.

Yes. In everyday speech, people often shorten it.

For example:

• 수업 끝나고 피아노 연습해요.
• 수업 끝나고 나서 피아노 연습해요.

These sound more conversational because Korean often drops particles like 이 and 를 when the meaning is clear.

The full sentence you gave is still perfectly natural and very good for learning, because it clearly shows the grammar:

• 수업이 as the subject
• 피아노를 as the object
• 끝나고 나서 as the after structure