The concept of "equivalence" in translation has not only been a principal problem among Western translation theories for 2000 years, but also a basic one in modern time.

翻译中的“对等”概念，不仅是二千年来西方翻译理论的一个根本问题，同时，也是现代翻译的根本问题。

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The heating reactor technology of our country is in the lead in the world. In order to spread the heating reactor, the principal problem we must solve is to keep the nuclear safety.

我国的低温核供热堆技术处于世界领先水平，为了进一步推广核供热堆，需要解决的首要问题就是保证核安全。

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Our principal problem is lack of time.

我们的主要问题就是缺少时间。

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In fact, the principal problem facing the two major parties in the congressional elections of 1998 was voter apathy.

CNN: By Ronald D. Elving, CQ Staff Writer

Early leaders recognized that the principal problem facing African-Americans was lack of access.

CNN: Lesson plan: Today's young leaders

Our undefined, unstable dollar caused the crash of late 2008, and it is the principal problem behind what is unfolding as the slowest economic recovery on record.

FORBES: What Is Gold Really Worth?