a. I am delighted to learn that you've got married on 21st May. I was delighted to learn that you had got married on 21st May.
b. Unfortunately I cannot come. I'll go to an international fair. Unfortunately I couldn't come. I went to an international fair.
c. I had to represent our company at the fair. I have to represent our company at the fair.
d. I am sorry I cannot come to this important event in your life. I was sorry I couldn't come to this important event in your life.
e. I've already bought a present for you. I hope you'll like it.
f. Please give my regards to your parents.
g. My best wishes to you and your husband!