he told us that so successful was his business that he was able to.为什么倒装

不是说so等词只有在狭义上的句首才会引起倒装,在广义的句首如宾语从句,主语从句,连词后面等等,都可能引起倒装.例如,It is a fact that so fast did he run that no one could catch up with him.再如He has been married for 40 years,and never has he quarreled with his wife.