2--It's our goal to make learning them "a piece of cake."
 
1--Everyone in America knows this idiom because it's so easy.
 
2--Hey! How did you do on the exam?
 
1--I got an A. The exam was a piece of cake.
 
2--In American English, when you say something is a "piece of cake," it means it is very easy to do.
 
1--In the 19th and early 20th centuries, African Americans started "cake walks."
 
2--They walked or danced around a cake in pairs, and the best couple won the cake.
 
1--This is how the idioms "cake walk" and "piece of cake" came about -- both meaning that something is very easy to do.
 
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注解：
 
1.idioms俚语 
2.centuries世纪 
3.idioms习语 

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译文：

1--你觉得美式俚语很难理解么？
2--我们的目标就是让你觉得学习它们就是“a piece of cake”（非常容易的事）。
1--美国人都明白这个短语，因为这很容易。
2--嘿！考试怎么样啊？
1--我得了A，考试很容易。（a piece of cake）
2--在美语中，当你说某事是“piece of cake（一片蛋糕）”，意思是它很容易。
1--在十九世纪和二十世纪初，非裔美国人开始走“蛋糕步”（cake walks）。
2--他们成双结对地围着蛋糕走或者跳舞，跳的最棒的一对赢得蛋糕奖励。
1--这就是“cake walk（蛋糕步）”和“piece of cake（一片蛋糕）”的来源——它们都是指某事很容易。