Your classmate has written to invite you to his or her birthday party. write back to him or her
giving
reasons
why you can't
attend the birthday party.

This question does not have a specific answer, nor does it elaborate further into the question, so I will provide an answer that may or may not be approximately what the question is asking.

Dear [classmate’s name],

I would love to attend your birthday party, but unfortunately I won’t be able to. The reason for this is because [state the specific reason why you are unable to attend the birthday party]. Happy Birthday!

From your classmate,

[your signature]

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