If he had not resigned, we would have been obliged to give him the sack. --> Had... to give him the sack.

If she decides to come, please telephone. --> Should ... telephone.

If he were to resign, it would cause a lot of problems for the company. --> Were ... a lot of problems for the company.

If he had understood the problem, he wouldn't have committed those mistakes. --> Had ... wouldn't have commited those mistakes.

If you decide to cancel the contract, please let me know. --> Should ... let me know.

If she were to find out that he was cheating on her, she would go mad. --> Were ..., she would go mad.

If I had known about those problems, I would never have moved here. --> Had ... moved here.

If you were to change your mind, I would be most grateful.--> Were ... most grateful.