I'm sure he didn't understand what I was saying to him. (misunderstood) He what I was saying to him.

Đáp án: He must have misunderstood what I was saying to him.

      be sure (that) someone did sth - someone must have done sth

      chắc chắn rằng ai đó đã làm gì

      be sure (that) someone didn’t do sth - someone can’t have done sth

      chắc chắn rằng ai đó đã không làm gì