I'm not sure she believes me. She probably just thinks I've gone off the deep end.
我不确定她是不是相信我。她可能只是觉得我神志不清了。
I'm not sure whether I should tell you this.
我拿不准该不该把这事告诉你。
I'm not sure that this is the most valuable use of my time.
我不能肯定我的.时间这样来安排最有价值。
I think this is their house, but I'm not sure.
我想这是他们家，但不敢肯定。
One should never criticize if one is not sure of one's facts.
一个人如果对自己掌握的事实没有把握，就绝不该随便批评。