"A boy swimming in the river" 表示男生在河里游泳。这是一个常见的英语表达，没有特殊的来源。
下面是一些英文例句，以及它们的中文翻译：
The boy was swimming in the river when he saw a fish swimming by. (男孩在河里游泳时，看到一条鱼游过。)
I saw a boy swimming in the river when I was walking by the bank. (我在沿河堤走路时看到一个男孩在河里游泳。)
The boy loved to swim in the river on hot summer days. (男孩喜欢在炎热的夏天游泳。)
总的来说，"A boy swimming in the river" 表示男生正在河里游泳，是一个描述一个正在进行的动作的常见英语表达。
下面是一些英文例句，以及它们的中文翻译：
The boy was swimming in the river when he saw a fish swimming by. (男孩在河里游泳时，看到一条鱼游过。)
I saw a boy swimming in the river when I was walking by the bank. (我在沿河堤走路时看到一个男孩在河里游泳。)
The boy loved to swim in the river on hot summer days. (男孩喜欢在炎热的夏天游泳。)
总的来说，"A boy swimming in the river" 表示男生正在河里游泳，是一个描述一个正在进行的动作的常见英语表达。