We are swimming ( ) the sea.为什么写across不用through?

We are across the sea Look!The man is swimming _______ the lake为什么用across而不用throug 为什么要用“across the room"而不用"through the room"? 是swimming through 还是swimming across? 穿过沙漠为什么多数与through而不用across? 英语,为什么53用through而不用across 连词造句 boys,those,the,are,river,across,swimming. 为什么坐船用by sea不用by the sea “go through the street”还是“go across the street",为什么? The ducks are swimming in the river.为什么用介词in,而不用on 有"across the sea"吗? We went swimming in the river,这句为什么swim不用过去式呀?