swim的现在分词为什么双写m

　　We cooled off with a swim in the lake.

　　在湖里游泳后我们感到凉快了。

　　Let's go for a quick swim in the pool.

　　咱们去游泳池游会儿泳吧。

　　I swim a mile a day.

　　我每天游一英里。

　　I never learned to swim and neither did they.

　　我从没学过游泳，他们也没学过。