swim ( )填across还是through

1、首先是词性的区别：across为介词,而cross为动词.
（动词为“穿过,横穿”,名词为“十字,十字路口”）
2、当然across必须与through 区别开来.across为“横穿”,与“道路”交叉形成“十字”.而through为在立体空间中的“穿过”.如：go through the forest“穿过森林”,go across the street “穿过大街”
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through表示“贯通、直穿、透过、穿过”的意思,即是从一头（边）贯穿到另一头（边）.
例The river runs through our city.这条河流经我们市.
He passed through the hall.他穿过大厅
across表示“横穿、横过、横渡、横跨”
例：I swam across the Changjiang River 20 years ago.
20年前我横渡了长江.
Look left and right before you go across the street.
过马路时要左右看.
through,across和over的区别
①、through：穿过
I threw it through the window.
Is it quicker to drive round the town,or straight though the centre?
②、across:from one side to the other
（在平面上）越过,在/到……对/面,横过.
I can’t swim across the river.
我不能游到河对岸.
He went across the hall to the door.
他越过大厅到门口.
③、over:翻越过
across和over两者都表示从一侧向另一侧的运动：
She drove over /across the bridge.
但如果越过的是一个高的物体,则用over:
He climbed over the wall.
如果越过的是一个平面,则宜用across：
He went across the hall to the door.
across和through都表示“穿过”,“通过”的意思,across表示从物体表面“穿过”、“通过”;而through则表示从内部“穿过”、“通过”.