I made a mistake.
Please accept my apologies.
I'm sorry. I didn't mean to . . .
(I'm) sorry. I didn't realize that . . . .
I'm sorry (I have) to tell you this, but . . .
I hate to tell you this, but . . .
I don't know how to tell you this, but . . .
I have some bad news.
We regret to inform you that . . .
Regretfully, . . .
Unfortunately, . . .
1. We still feel sorry for the trouble that has caused you much inconvenience.
2. We regret to inform you that we are not in a position to enter into business relations with any firms.
3. Please accept our deepest apology for any inconvenience this matter has caused you.
4. We are sorry for the short delivery. 我们为交易量短缺像贵公司致歉。
5. We are terribly sorry to learn from you that the quality of goods is not satisfactory to you.