格式自己写把
DearJack,thankyouverymuchforinvitingmetoyourbirthdaypartyintheweekend.However,IamverysorrytotellyouthatIcannotmakeit.
MymotherisverysickatthismomentandIhavetotakecareofmymotherathersickbed,alsoIhavetodothehouseworkplusthehomework,whichtakeupalmostallmyfreetimeintheweekend.
Inaddition,therewillbeanEnglishexamonnextMonday,andsincemyenglishlevelisnotsogood,Ihavetopayextraeffortandtimetoworkonitsotoachieveahighscore.
AlsomyfatherhadgonetoanimportantmeetinginShanghaiandhewillonlybebackonnextWednesday.Icannotleavemysickmotherunattended.
IamverysorrythatIcannotgotoyourbirthdayparty.Hopeyouhavefunduringtheparty.