Because we have to go to a school.
> We hate Monday because we need to go to school. 

 Also we have to wake up early.
> This is correct.

Whwn we go to a school, we have math class and Krean class.
> We have Math class and Korean class every Monday and they are my least favorite subject.

So we hate Monday.
>This sentence is correct.

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Hello there Eric!

      Thank you so much for doing  your homework. Now I understand why you don't  like Mondays. I added some words in your sentence and rephrase some to make your thought clearer.

   Anyway, you did great and I always admire you in the class. Keep excelling and study your book! ^^ Teacher loves you. 

~Teacher Cassie