I do not know, for certain, what they mean.

我不知道他们所说的确切含义。

youdao

What I do know for certain, however, is that one night, a night when he and I both needed it most, I made him laugh, and that makes me proud.

我只知道一件事：那就是今晚，是我们俩都最需要的一个晚上，我让他笑了出来，而那使我骄傲。

youdao

I do not know for certain what the future holds but I am confident that with hard work, a little bit of luck and a supportive network of friends, I can be as successful as I want to be.

我不确知未来，但我相信通过艰苦的工作加上一点点的运气和朋友们的帮助，我一定能够心想事成！

youdao

No one can say for certain how this change will end, but I do know that change is not something that we should fear.

WHITEHOUSE: President Obama Speaks to the People of Brazil