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

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

我不知道他们所说的确切含义。

youdao

From a certain perspective that I really do not know where is the future, but differs from the previous, but different only for time.

从某一角度说我真的不知道未来在哪里，但又和以前不同，但不同的也只是时间而已。

youdao

The scientists do not yet know for certain how the gene works.

BBC: Anthrax antidote hope

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