But he added, he did not know for certain.
但是他补充说，他也不确定。
Mr Suttles said he "did not know for certain" why it had failed.
Suttles先生说他“并不十分确定”前种方案失败的原因。
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！
I do not know, for certain, what they mean.
我不知道他们所说的确切含义。
I do not know, for certain, what they mean.
我不知道他们所说的确切含义。
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