汉译英：严禁十五岁以下的小孩在河里游泳

Prohibit under 15 swimming in the river

Children under 15 is prohibited/forbidden to swim in the river

Children under 15 swimming in the river is prohibited/forbidden

都可以。一楼为什么要用过去被动式呢？

PEOPLE UNDER 15 WAS FORBIDDEN TO SWIM IN THE RIVER

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温和一点的
Don't swim in the river with the age under 15.

强硬一点的
No swimming in the river with the age under 15.
Forbiding swim here for under 15 ages people.