- [Casual]
Sorry, I cannot make it.
[Polite]
It is my honor to be invited to the dinner though, I would not be able to join it.
- [Casual]
I cannot take your offer.
[Polite]
I appreciate your kindest offer though, I would not be able to accept it.
- [Casual]
I cannot accept your proposal.
[Polite]
Your proposal looks very attractive though, I am afraid I have to decline it.