①I had studied math from last year, so I could pass an exam.
②He hadn't drunk until he became 20 years old, thereby he spent happy life.
③If I had studied math from last year, I would have passed an exam.
④If he had drunk until he bacome 20 years old, he would have spent happy life.
間違っていたら申し訳ないです！
③④は仮定法過去完了を使いました。