un over:复查 That child was run over by bus. 那个小孩被公共汽车压倒. Once there,you can run over him. 然后您可以从他头上跑过去. run across:偶然遇见 Don't run across (the road). 不要横越马路. You must not run across the road. 你不要跑过马路.
看这里关于更多Over and Above, Below and Under的解释讨论: http://www.***.com/forums/index.php?showtopic=511 Over 是有跨过某个表面的意思。不接触表面。 Across 穿过的意思。比如run across the road.