across意为“横穿，穿越”，强调穿过某一平面，表示运动发生在物体的表面。

The Great Wall winds its way from west to east ,across the deserts,over the mountains,through the valleys,till at last it reaches the sea.

长城从西到东，穿过沙漠，越过高山，穿过深谷，蜿蜒而行，最后到达海边。

He can swim across the river.

他能游过这条河。

Be careful while walking across the street .

过马路小心点。

through意为“跨过，越过”，强调从某一空间内通过，表示运动发生在某物的空间内。

Walk through the gate ,and you'll see the rest room.

穿过大门，你就会看到洗手间。

They drove through a snowstorm.

他们驱车穿过暴风雪。

They walked through the forest.

他们穿过了森林。