swims。swim为谓语动词，只能作句子谓语。swimming为非谓语动词，属于非谓语动词中的动名词一种，具有名词的特性，可以作句子主语、宾语，以及定语，还有表语等。

swim例句

He can swim well.

他游泳游得很好。

He has swum the English Channel three times.

他已三次游过英吉利海峡。

He was hot and tired and his head was swimming.

他又热又累，头晕目眩。

He goes for a swim every morning.

他每天早晨去游泳。

swimming例句

Swimming was one of her greatest satisfactions.

游泳是她最大的乐趣之一。

He broke several world swimming records at the 1996 Olympics.

他在1996年的奥运会上打破了好几项世界游泳纪录。

The fish dive down to about 1,400 feet and then swim south-west.

鱼下潜到大约1,400英尺然后向西南方游去。

He landed two 5 lb chub from the same swim.

他在同一个深潭钓了两条五磅重的圆鳍雅罗鱼。

Fish attempt to swim through the mesh and become entangled.

鱼试图游过网眼，结果被网缠住。