Six little ducks that I once knew; 我曾经见过六只小鸭子。
Fat ones, skinny ones, fair ones too; 有的胖，有的瘦，有的不胖也不瘦。
But the one little duck with the feather on his back; 但是有一只鸭子背上长着一根羽毛；
He led the others with a quack, quack, quack, 嘎嘎地叫着带领其他小鸭。
Quack, quack, quack,
Quack, quack, quack,
He led the others with a quack, quack, quack.
？

1. Once, 以前/曾经
He was once a prince. 他以前是个王子。

knew, 是动词know的过去式
I don't know how to play this game. 我不知道怎么玩这个游戏。
He knew he was wrong yesterday.他昨天就知道自己错了。

fat, 肥的
skinny, 极瘦的
fair, 合理的，这里指：鸭子不胖也不瘦

feather, 羽毛
Peacocks have beautiful feathers.孔雀有漂亮的羽毛。

on his back,在他的背上
The little duck has feather on his back. 这只小鸭子背上长出羽毛了。

6. led, 是动词lead的过去式，表示“带领，领导”的意思
He led the others with a quack, quack, quack. 他领着其它小鸭子们嘎嘎地叫。