Oh, I'm sorry - I didn't see you there.
Tom, I'm really/so sorry about last night - it was all my fault.
Hi! Yes, of course I can go for dinner tomorrow, but I [will be] a little late, sorry about that. (Skills for Life (Entry 1); A1; Polish)
I'm sorry (that) you had such a difficult journey.
We were both sorry to hear you've been ill again.
I was sorry that you couldn't come to my birthday party. (Key English Test; A2; Japanese)
I feel so sorry for the kids - it must be really hard for them.
Anyway, I feel sorry for her and I wonder: what would I do if I were her? (Preliminary English Test; B1; Vietnamese)
I'm sorry but I think you've made a mistake.
I'm sorry, I don't agree.
I'm sorry, but I just don't think it's a good idea.
Sorry, you can't go in there.
I am sorry but I can't tell you the starting date right now, [although] I think it's around [the] 15th of July. (First Certificate in English; B2; Hungarian)