I'm sure that you're very handsome!中文释义为“我确定你非常英俊”I'm sure意为“我确定...”或为“我坚信”.that引导后面的从句.
I’m not sure意思相反.
sure 是形容词,意为“肯定的,当然的”.常用来回答一般疑问句,等于 Yes或Certainly.sure的用法：
（1）be sure＋不定式.常用于祈使句,表示要求,意为“务必、请一定”.指外界的看法.例如：
I think he's coming,but I'm not quite sure.
Be sure to telephone me and give me all the news.
（2）be sure＋of或about.意为“相信,对……有把握”,后接名词、代词或动词的-ing形式.常用来表示“人对某事的看法”,指主观上的看法.例如：
Are you sure of that meeting?He is sure about this answer.
（3）be sure＋从句.表示“某人对……有把握”.例如：
I'm sure that he is coming to help me.
The teacher is sure that these books are interesting.
（4）make sure意为“务必、确信”,其后也可接of或about或动词不定式或that从句.例如：
I made sure (that) he was badly ill.
Will you make sure of his return?
Make sure to come to the party on time.