Along with them goes social mobility,ambition to rise in the urban world,a main factor in bringing down the births in Europe in the nineteenth century.

参考答案:

要点：这是一个完全倒装句，主语是social mobility,ambition to rise in the urban world,a main factor in bringing down the births in Europe in the nineteenth century，谓语动词是goes。ambition to rise in the urban world是对social mobility的进一步说明。a main factor在意思上相当于which is a main factor in bringing down the births in Europe in the nineteenth century，修饰ambition to rise in the urban world。

译文：伴随他们而来的是社会的流动性，人们强烈地希望在城市里发展，这是19世纪使欧洲出生率下降的主要因素。