当你仰头看天时，天空高高在上。一架飞机进入你的视野，从你看到的天空的下面呈直线从你的视野的一侧到另一侧然后消失看不见了。你看到的就是 A plane flies across the sky. 

(1)如果是“纵向穿过” 就使用介词“through”

The burglar got in through the window.盗贼是从窗户进来的。

The sand ran through my fingers.沙子从我的手指缝间漏了下去。

The path led through the trees to the river.这条小路穿过树林通向河边。

The Charles River flows through Boston.查尔斯河流经波士顿。

The doctor pushed his way through the crowd.医生挤过人群。

(2)如果是“横向穿过”就使用介词“across”

He walked across the field.他走过田地。

I drew a line across the page.我在这一页上画了一条横线。

A grin spread across her face.她粲然一笑。

Where's the nearest bridge across the river?过河最近的桥在哪儿？