1 If I weren't too busy, I should go to the concert.
If I hadn't been too busy, I should have gone to the concert.
2 If I came home late, I should be very sorry.
If I had come late, I should have been very sorry.
3 If he received my letter, he would be very happy.
If he had received my letter, he would have been very happy.
4 If she didn't come in time, we shouldn't have to wait for her.
If she hadn't come in time, we shouldn't have had to wait for her.
5 What should we do if they were late?
What should have we done if they had been late?