这个美国谚语怎么说：当一个人知道自己要什么时,整个世界也将为之让路

Whenever we are doing something that fills us with enthusiasm we are following our legend.

When you really want something, the whole world conspires to help you
第二句是你要的答案！

楼上正解，也可以说：
When you really really want something, the whole world conspires with you in achieving it.
或
When you really want something, the whole universe conspires to get what you want.
自我感觉第二句好些~两个want显得比较有水准~~

当一个人知道自己要什么时,整个世界也将为之让路
The world will give his way when a man do know his want .
That a man really knows his want is overwhelming throughout the world.