It was difficult for him to catch up with his classmates because he was too lazy. => If

$\text{=> If he had studied hard, he would have caught up with his classmates.}$

Đây là cấu trúc If loại 3: 

 Công thức: If + S + had + PP, S + would/might/could+have + PP

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`~rai~`

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