to sweep of fields and mountains is to grasp
不定式做主语~
to do A is to do B 见过的吧?
做的时候找主谓 可以先看
to start blabla 介词短语省略
as blabla 状语省略
at blabla 介词短语作状语省略
(sweep of fields and mountains) is (to grasp )the vastness of the ancient Persian Empire
is没画线 而且句子为简单句 所以为谓语
is前面动词词组的不能作为主语
参照is后面的 就可以得到to sweep.了