I'm not sure she believes me. She probably just thinks I've gone off the deep end.
我不確定她是不是相信我。她可能只是覺得我神志不清了。
I'm not sure whether I should tell you this.
我拿不準該不該把這事告訴你。
I'm not sure that this is the most valuable use of my time.
我不能肯定我的.時間這樣來安排最有價值。
I think this is their house, but I'm not sure.
我想這是他們家，但不敢肯定。
One should never criticize if one is not sure of one's facts.
一個人如果對自己掌握的事實沒有把握，就絕不該隨便批評。