翻译in order to become skill performer,the student must become

翻译in order to become skill performer,the student must become
这个句后一句 型要怎么翻译?in order to become skill performer,the student must become proficient at using the units of lanuage.And the unit of a language is not ,as was once commonly supposed,the word,but the sentence.

这是意译：为了成为一个skilled performer,学生必须熟练掌握语言单位的运用.而语言的单位其实指的是句子,而不是人们通常以为的词汇.
第二句的主句是The unit of a language is not the word but the sentence.不是the word而是the sentence,中间插入了一句as was once commonly supposed用来补充说明曾经人们以为单词就是the unit of a language.