Hello Woojung,

Thank you for submitting your homework.. Wow so you are really good at playing piano more than your brother. Maybe I will ask his opinion about it.. ahahaha...

Please take notes of my corrections. Thank you..

I like to play piano. -Correct

When I'm playing the piano, it feels like my finger moving by itself and feels happy, and great.

Correct: When I play the piano, it feels like my fingers are moving on their own. I feel happy and great.

** Use a present simple tense when talking about regular activities or facts.
** On its / their own = unaccompanied by others or alone.
** Use a period (.) to form a declarative sentence.

My brother thinks he does really great at piano but in real he can only do airplane well.

Correct: My brother thinks he's really good at playing piano but it's not. I think his forte is doing something that is related to airplane.

** Use a period (.) to form a declarative sentence.
** Be careful with the word order.
** Use "forte" to mean a strong ability or something that a person can do well.

He is really bad at piano.

Correct: He cannot play piano well.

** Use "cannot", the negative form of can,  to mean not able to do it well.
** Use "well" to mean in a good way, to a high or satisfactory standard.

I learned piano nearly 3 years and I can do well but he only learned 3 or 4 months because he was bad at piano.

Correct: I had learned playing piano for 3 years that's why I can play it well unlike my brother. He had just learned for 3 or 4 months because he was not good at it.

** When talking about the past, we use the past perfect to go back to an earlier past time, to refer to something that had already happened or had been happening.
** Use "unlike" to mean different from (something/ someone).