1-the development of modern cities,both in population size a

第一句：the development of modern cities,both in population size and in functionality,necessitates the removal of certain existing structures whose history can often be traced back to centuries ago.
1）whose history can often be traced back to centuries ago 是修饰 existing structures 的定语从句,whose history 等于 the history of the existing structures.
2）表达的意思是：在群体规模和所起的作用两个方面,现代城市的发展需要一定的其历史能够经常追溯到几百年前的现有建筑物.
第二句：in the first place,new constructions must be proposed and carried out to accommodate the markedly-increasing urban population.
1）句子的意群应该划分为——
new constructions / must be proposed and carried out / to accommodate the markedly-increasing urban population.
其中——
句子主体为：new constructions must be proposed and carried out
后面的不定式to accommodate the markedly-increasing urban population 是目的状语.
2）英语常用被动语态,这一点在汉语中却很少见,因此如果用汉语表述的话,应该将其变通为主动语态,翻译如下：
首先,（我们）必须规划新的建筑物,同时为适应显著增长的城市人口将其贯彻落实下去.