Sometimes that standard is quite difficult, if not impossible, to achieve.

有时那个标准，即使并非不可能，也是很难达到的。

《柯林斯英汉双解大词典》

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That lack of curiosity produces a relative lack of knowledge, and the lack of knowledge is difficult if not impossible to compensate for later on.

好奇心的缺乏导致了相应知识不足，而知识的缺乏即便可以弥补，但真正做到也是很难的。

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For most men and women, fitting in an hour of moderately intense exercise in an already hectic day is difficult, if not impossible.

VOA: standard.2010.03.30

but he's also insisting that this is an incredibly difficult, if not impossible, moment of thought.

但他同时坚持，如果可能，这对思维来说也是一个极其艰难的时刻。

耶鲁公开课 - 文学理论导论课程节选

If it's not clear, the problem is going to be impossible to solve.

问题也就不可能，得到解决。

麻省理工公开课 - 热力学与动力学课程节选

The implications of the way people vote are difficult, if not impossible to foresee.

BBC: Everyone has two votes - although you don't have to use both

Change would be difficult, if not impossible, for processes locked into traditional on-site IT systems.

FORBES: Cloud Computing Now Makes It Easier (and Cheaper) to Innovate: Study

This film would have been hard, if not impossible, to make just a few years ago.

CNN: Review: Plenty of bite in 'Deep Blue Sea'