① If I had studied in my youth, I should be living a happy life now.[条件が過去で帰結が現在]

= As I didn’t study in my youth, I’m not living a happy life now.

② I could show you the pictures if they had not been stolen.[条件が過去で帰結が現在]

= I cannot show you the pictures because they were stolen.

③ If he were ill, we should have heard by now.[条件が現在で帰結が過去]

④ He would have passed here at about eight o’clock if he was expecting to get to school at nine.

[条件が現在で帰結が過去]