Apology Letter For Borrowing Without Asking
Dear {Recipient},
I would like to offer my sincere apologies for borrowing your {item} without asking your permission first. I failed to practice common courtesy and completely understand why this incident left you feeling {angry/annoyed/disappointed/worried}. I would react the same way if I were in your shoes.
Please know that I have since returned your {item} in the same condition that I found it in. Again, I'm sorry for my oversight and I promise this won't happen again.
Regards,
{Your Name}