I‘m not sure.我不敢确定。

I’m not sure what you mean.我弄不清楚你想说什么。

I don’t know for sure.我不太清楚。

Are you sure?真的吗？

I’m sure I can do it.我确信我能做好。

（Are you sure you can do it?/Yes,I’m sure I can do it.你真的可以做吗？是的，我确信我能做好）

He is sure to succeed.他一定会成功。

Please make sure that the house is locked before you leave.请你确信锁上房子后才离开。

Do you feel sure about it?你对这个有把握吗？

Be sure not to forget it.千万不要忘记呀！

Sure you can!你肯定能行！

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polite/plain/progress/supply/approve/option/people/hope/help/keep
You should be polite to everyone.你应该对每个人都礼貌。
You have made a lot of progress with your English.你的英语有了很大的进步。
You have no option.你没有选择的余地。
There are three options open to us.我们有三种选择。
A.Keep it up.就这样，坚持下去！B.Thanks for the encouragement.谢谢你的鼓励!