不确认英文怎么写

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。

Ask me if you're not sure how to do it.

你要是拿不准怎么干，就问我。