Write a letter to your previous English teacher because you can't go to his/her invitation.

Dear Sir,

Thank you so much for inviting me to a dinner at your home this Thursday. Unfortunately, I would not be able to make it because I am leaving for Australia on Thursday morning.

You have not just been my English teacher but also my friend, philosopher and guide throughout my school days. It was you who motivated me to go abroad for my higher studies and it was because of your guidance that I cleared my IELTS with such good bands that I got into one of the best colleges of Australia. It is also very kind of you to invite me for dinner and it is my loss that I cannot come. Actually, my ticket has been already booked. My flight is leaving on Thursday night and I have to leave from here for Delhi that morning.

Sir, I take this opportunity to thank you once again for all you have done for me and I shall keep in touch with you after going there through e-mail. My e-mail ID is kiran@yahoo.com.

Yours faithfully,

XYZ