Thank your invitation.I would like to join in your birthday party.But my mother is ill, I have to look after her, I could not join in your birthday party.It's a pity not to take part in your birthday party to celebrate your birthday happy. I am sorryI have to say happy birthday to you by writing a letter.I have been prepared a gift for you.I will send to you by EMS.I want to say:Happy birthday to you,Tom.