That's a hard / difficult question.
那是难以回答的问题。
It is hard (difficult) for him to teach English.
对他来说，教英语是件力不从心的事。
The main subjects for ceramic machining researches include the hard-difficult machining theory, the grinding mechanism, precision grinding and high efficiency grinding.
包括工程陶瓷材料难加工的理论基础研究，磨削机理，超精密磨削，高效磨削加工等方面的研究动态。
The main subjects for ceramic machining researches include the hard-difficult machining theory, the grinding mechanism, precision grinding and high efficiency grinding.
包括工程陶瓷材料难加工的理论基础研究，磨削机理，超精密磨削，高效磨削加工等方面的研究动态。
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