1. A Piece of cake 小菜一碟

No, when someone says that the assignment they just finished was a piece of cake, it does not mean that their professor gave them a red velvet cupcake for their midterm paper, what a piece of cake actually means is that something is very easy to complete.
不，当某人说他们刚刚完成的任务就像一块蛋糕，那么它并不是说教授因为他们的期中考试成绩奖励他们一份红丝绒蛋糕，“a piece of cake”实际是指某事很简单很容易完成。

2. Costs an arm and a leg 花了我一大笔钱

It would be a strange world we lived in if buying that fancy shiny purse literally required us to chop off our body parts to give as tribute to the Louis Vuitton gods. When something costs an arm and a leg it actually means that something is very expensive.
如果为了买我们心爱的闪亮的包包，比如LV，却要让我们砍掉手脚以示敬意，那这个世界未免太奇葩了吧。当某物花掉你的胳膊和腿时，它实际是指这个东西超级贵。

3. Break a leg 祝你好运

Oh, look, another idiom about legs. You’re about to take your dreaded calculus final and before you head into your classroom your roommate texts you, “Break a Leg!”  Why, you think in your head, would he ever wish that upon me? I thought we were cool with each other. Well, your roommate surely doesn’t want your bones to break while walking to your seat in the exam room that’s for sure. Break a leg actually means good luck!
哦，快看，又一个与腿有关的习语。你马上要参加你最害怕的微积分期末考试了，在你进入考场教室前你的室友给你发来了一条短信：”Break a Leg!” 什么，你在脑中暗暗的想，难道他希望我断胳膊断腿？我一直以为我们关系很好呢。那么，事实上你的室友并不是希望你在走入考场位置时摔断骨头，这是肯定的。Break a leg实际上就是“祝你好运”的意思！

4. Hit the books 用功读书

If you’re a student in an English speaking environment you’re probably going to be hearing this phrase a lot. Before you imagine students running into their campus library and punching, kicking and wrestling apart the complete works of Shakespeare, we would just like to say that hit the books actually means to study. There there, you can still punch books in your spare time if you want, we won’t judge you.
如果你是一个生活在英语环境下的学生，你肯定会经常听到这句话。当你脑子里在歪歪各种学生冲进校园图书馆拳打脚踢撕扯莎士比亚著作的画面时，我要告诉你hit the books实际上指的是去学习（用功）。当然当然，如果你想，你还是可以在闲暇时间猛捶你的书，我们不做评论。

5. Hit the nail on the head 恰到好处，一针见血

This idiom has to do with doing or saying something that is precisely right. If you don’t understand this, just think about that sweet feeling you get when you swing a hammer at a nail and hit it perfectly.
这个习语表示做某事或说某事恰到好处。如果你还是不懂，那么就想象一下当你挥舞着铁锤恰到好处地正中要害，一锤定音，那种感觉有多美妙。

6. You can’t judge a book by its cover 不要以貌取人

How many awesome books do you think you’ve never read in your life just because the cover did not catch your eye? This idiom does not only apply to books however, but can be used for everything in general. Essentially it means that you should not decide upon something based just on outward appearances.
你能想象下仅仅由于封面不够吸引眼球，你错失了多少本好书吗？这个习语不仅仅适用于书，还可以广泛的应用于很多场合。总体而言，它是指不要光从事物的表面现象来做决定。

7. Bite off more than you can chew 贪多嚼不烂，自不量力

Imagine your waiter brings you the biggest juiciest hamburger from your favorite American restaurant. In your hunger, you grab it quickly and take a giant bite out of it. Unfortunately, the bite you’ve taken is too big, and you end up looking like an idiot trying to shove this bite down your throat while drinking water and trying not to choke. That is the most literal sense of the meaning, but in general it just means to attempt to take on a task that is too much for you to handle.
想象一下在你最喜欢的美国餐厅，服务员为你带来最大最美味的汉堡包。饥肠辘辘的你，抓起汉堡狠狠地咬了一大口。很不幸，你咬的这一口太大了，以至于你最终的结局是一边想要艰难努力的咽下这一大口食物，一边又喝水以防呛到自己，场面是不是像个白痴一样很傻很可笑。

8. Scratch someone’s back 投之以桃，报之以李

We all know how difficult it is to scratch that itch on your back that your hand just aren’t flexible enough to reach, so why would you want to scratch some random person’s smelly back? Because if you do, they may eventually be willing to scratch your own smelly back when you need it! What this idiom means is to help someone out with the assumption that they will return the favor in the future!
我们都知道想要给后背挠痒是件很麻烦的事，因为手臂没有那么灵活，那么你为什么还要偶尔给别人的后背挠痒呢？答案是，如果今天你帮了别人，那么某一天也会有人愿意帮你的后背挠痒！这句习语的意思就是预支自己对他人的帮忙，以防日后自己不时之需时可以有人情债可收。