I found him to be a diligengt student.
应填 to be.
本题主要看found后面的代词是主格还是宾格.
1、I found him to be a diligengt student （to be a clever boy是him的宾语补足语.句型：find sb/sth ( to be )+adj./n./adv.觉得某人或某物.）
2、I found he was a diligengt student.(found后接宾语从句,从句谓语动词was 与主句的时态一致)