I am not quite sure whether I am free.
I am not sure.
Maybe. Maybe not.
It looks as if I am not free tonight.
I am not free at all.
I am just remember that I have got another appointment.
Sorry .I am afraid I can not go to the theatre tonight.
I am away tonight.
Sorry I can not .I have got a terrible toothache now.
I am really a little bit busy for the next few weeks.
I will give you a ring one of these days .
I would love to ,but I am afraid I will be quite busy then.
I will come round to other place.
I am afraid I am busy on Sunday.
It might be a little early.
That is difficult for me.
I have to visit a friend tonight.
Thanks for the offer, but I am afraid I have an essay to finish.
It is very kind of you to invite ,but I am afraid I am busy tonight.
Well , tonight is not really convenient .How about next Friday?
I would love to , but I do not suppose I have time.
It is very nice of you to ask, but honestly, I really would not be able to.
It is very kind of you to invite me, but I have got a previous engagement.
I would love to go ,but I do not think my husband will make it as he's schedule d to leave for a conference the day before the occasion.
I regret we are unable to accept your kind invitation.
You know I could not possibly make it as I will be away on a business trip in New York.
You know I have already been asked to join my family for the weekend.
It is unlike we will be back before the day you suggested .In view of this ,we are unable to accept your invitation.