the population is large and about four fifths of it are farmers

问题描述:

the population is large and about four fifths of it are farmers

人口众多,约五分之四的农民

and 连接两个简单句 他们是独立的
可以看成the population is large。about four fifths of it are farmers

population 做主语时,谓语一般用单数,如:
the population 【is】 large
但如果population 和分数,百分数连用,做主语,表语是复数时,此时谓语一般用复数,如:
about four fifths of it(=the population) 【are】 farmers(复数名词作表语)