retained by... 仅仅是一个过去分词短语。组成句子的话，需要还很多成分。例如：

1. The money was retained by him. （构成被动语态）那些钱又他保留。

2. The money retained by him was lost. （作 the money 的定语） 他保留的那些钱丢了。

网友这句话：

The prolonged connection with France retained by its new rules and landlords made a deep impression on the English language. 

retained by its new rules and landlords 是 The prolonged connection with France （与法国之间的长久关系） 的定语。与句2的作用相同。