Question

I'm not sure和 I'm not certain有区别吗？

Answer 1

.sure 的主语只能是人，certain的主语人或物均可．
I am sure of(相信） it. I am sure that she will be here on time.（相信）．　He is sure to arrive on time.(一定）．以上三句均可用certain代替．
２.It is certain that-clause,不可以用sure.如：It is certain that more and more Chinese are learning English, and that not all can go further.
3. John sure is living in Shanghai now.sure 用在任何谓语动词之前，意义为确实．

Answer 2

sure 的主语只能是人，certain的主语人或物均可．
I am sure of(相信） it. I am sure that she will be here on time.（相信）．He is sure to arrive on time.(一定）．以上三句均可用certain代替．
2.It is certain that-clause,不可以用sure.如：It is certain that more and more Chinese are learning English, and that not all can go further.
3. John sure is living in Shanghai now.sure 用在任何谓语动词之前，意义为确实．