学生疑问:
老师答疑:when是引导的时间状语从句,从句主干是 the Union Pacific Railroad began to build; westward from the Centre Plains city of Omaha(状语); to meet the Central Pacific Railroad(不定式表目的); advancing eastward from California through the formidable barrier of the Sierra Nevada(后置定语)。
点评分析:该句子的难点在于when引导的从句中修饰成分比较多,需要我们根据标点符号、介词短语、分词短语等划分句子结构。