Hello there Paul,
Belated happy Chuseok. Thank you for submitting your homework. Please take note of my corrections. I hope you enjoyed your holiday. See you in our class..
My chuseok was very nice day,because I met my cousin and grandfa and grandma.
Correct: My Chuseok was a very nice day, because I met my cousin, grandpa and grandma.
**Capitalized the first letter of the proper noun. (ex: Chuseok)
** Use "a" before a noun to mean one thing or person.
** Avoid repeated words in a sentence to not sound redundant.
** Be careful with the spelling of the word " GRANDPA".
I ate on chuseok delicious foods.
Correct: I ate delicious food during Chuseok.
** Be careful with your word order.
** Use " during" to mean "at a particular moment in a period of time".