‖ It was in America,therefore,that the great advances in nineteenth century agriculturalmachinery first came.that在句中起什么作用,

‖ It was in America,therefore,that the great advances in nineteenth century agriculturalmachinery first came.that在句中起什么作用,
‖ It was in America,therefore,that the great advances in nineteenth century agricultural
machinery first came.that在句中起什么作用,

‖ It was in America,therefore,that the great advances in nineteenth century agriculturalmachinery first came.that在句中起什么作用,
这是一个强调句型的结构,是It is/was...that ...的句型,中间的therefore是一个插入语,被强调的部分是地点状语
句子的意思是：因此,美国在19世纪工业机械化得到了很大的发展
再如
It was in the street that I came across my teacher.我是在街上遇到我的老师的