I am writing this letter to apologize that I could not attend to your graduation ceremony on the 31st instant(應(yīng)該使用副詞). Had I payed(改成paid) more attention to my plans, I would not have made such a stupid mistake. The primary factors for my absence are as follows.

　　First and foremost(不建議用這些字?jǐn)?shù)多的連詞), I will have to take part in a Chinese-speaking Contest held in my university on that day. Therefore, I could not be there with you and enjoy the great pleasure it brings about. In addition, I also need to take the English test in the afternoon.

　　Last but not least(不建議用這些字?jǐn)?shù)多的連詞), if I take all the factors into due consideration in advance, I would be able to spare my time for you. I really hope you will accept my sincere apology. Wish you enjoy the wonderful time.