YOU SAID:

I have to go to school and then go to a birthday party for my pet

INTO JAPANESE

私は学校に行ってからペットの誕生日パーティーに行かなければなりません

BACK INTO ENGLISH

I have to go to school and then go to my pet's birthday party

INTO JAPANESE

私は学校に行ってからペットの誕生日パーティーに行かなければなりません

BACK INTO ENGLISH

I have to go to school and then go to my pet's birthday party