$\begin{array}{I}\text{It was difficult for him to catch up with his classmates because he was too lazy.}\\\text{->If he hadn't been too lazy,it wouldn't have been difficult for him to catch up with his classmate.}\\\text{Điều kiện loại 3(không có thật ở QK):If S+had+P2,S+would/could/might+have+P2.}\end{array}$
