He mengxiong and other comrades demonstrated through the streets in two trucks decorated with red flags and streamers on which were written the words"Labor Is Sacred."
2.这里on which were written the words…可以写成：The words…were written on which……这个语序对吗？
Lying across the river is a bridge built two centuries ago. 这个句子可以写成这样吗：A bridge built two years ago is lying across the river
1. 关系副词where相当于介词+which，但这里如果使用where，语义不明确，容易造成误解，没有on which明确。
2. 本来的语序是the words “Labor Is Sacred” were written on which，on which引导定语从句，所以要提前；加之避免头重脚轻，主语后置，于是构成了全部倒装的句式：on which were written the words “Labor Is Sacred”（即方位状语+被动式谓语+主语）
3. Lying across the river is a bridge built two centuries ago. 是现在分词放在句首的倒装句，但这个句子里的现在分词lie（静态动词）并没有现在进行的意思，如果使用陈述语序则为A bridge built two centuries ago lies across the river. 显然改后的句子没有之前的句子描写生动。