[Your letterhead, if desired; if not, your return address]
[Date of letter—month, day, and year]
[Recipient's first and last names]
[Street or P.O. box address]
[City, State ZIP code]
Dear [recipient's name]:
I am writing to express to you how sorry I am for lying to you and telling you that I had completed all of the household chores that you had asked me to do, even though I knew that I had only completed a small part of them. It is really out of character for me to be dishonest or insincere like that, and I hope you will forgive me. In the future, I promise that I will be straightforward and honest with you, and I hope your trust in me will be restored.
Also, by the way, I have now completed all of those chores. I would like to do some extra chores this week as a way to show you how sincerely I regret making this mistake, so please let me know what tasks you would like me to do. Thank you for listening and for graciously considering my apology.
[Sender's first and last names]