be sure to do 与 be sure doing 都可以说,但语义上有区别:
be sure to do sth 意思是“肯定会做某事”,不定式所表示的动作尚未发生。如:
He is sure to come. 他肯定会来。
He is sure to win. 他肯定会赢。
be sure doing sth 意思是“肯定在做某事”,doing sth 所表示的动作正在发生、已经发生或为通常情况(此用法主要见于美国口语)。如:
He is sure reading in the room. 他肯定在房间里看书。
You are sure wasting your time. 你肯定是在浪费时间。
He is sure spending too much on it. 你肯定在上面花费了不少。