three added to seven makes ten

这个是动词的ED形式表被动的含义,be动词是可以被省略的,写成three is added to seven 也是正确的.
被动语态
英语的语态是通过动词形式的变化表现出来的.英语中有两种语态：主动语态和被动语态.
主动语态表示主语是动作的执行者.
例如：Many people speak Chinese.
谓语：speak的动作是由主语many people来执行的.
被动语态表示主语是动作的承受者,即行为动作的对象.例如：Chinese is spoken by many people. 英语English是动词speak的承受者.
1. 定义：被动语态,即不知道动作执行者或强调动作承受者的一种语态.例如中文常说：我被他打,这就是一种被动.但有时由于句子结构上的需要也要用被动,例如It is not unusual for workers in that region to be paid more than a month it 在句中作形式主语.而不定式to be paid more than a month是句子的逻辑主语.结合选项全句的意思是：“那个地方的工人一个多月后才得到工资是常有的事”.
2. 构成：be+past participle(过去分词)(简称P.P.)（+by+动作执行者）（过去分词的概念见上课）
3. 当主动语态要被改成被动的时候,我们把原句的宾语提前,作为改句的主语,主语后置,作为宾语.因此有一点要注意,不及物动词由于不加宾语,没有被动形式,但不及物动词如果与某些介词构成介词短语,可以用被动.例如The fire had been put out before the fireman arrived. Put是不即物动词,但put out是及物动词
4. 应用到各种时态和句型,如下：
① 一般式（一般现在,一般过去,一般将来）：am, is, are, was, were, is going to be , will be+done.ie. Once environmental damage is done, it takes many years for the system to recover. 本句的意思是：“环境一旦遭到破坏,需要多年时间才能恢复过来.”do作为及物动词有“引起,产生”的含义,do damage的意思是“造成破坏”.主语damage是及物动词do的动作对象,谓语应当用被动语态.同样的,还有,I will mend the machine.相当于The machine will be mended （by me）.
②进行时（现在进行、过去进行、将来进行）：be+being+P.P. ie.The classroom is being cleaned.
③ 完成时（现在完成、过去完成、将来完成）：have/has been + P.P.：例如The machine will have been repaired by 3 o’clock this afternoon.再如：My homework has been finished.
④ 其他时态 依词类推,可得到结果.
⑤ 情态动词的被动语态：主语 + 情态动词 + be动词 + 动词过去分词,例如Your teeth must be brushed.
⑥ 不定式的被动语态：to be done例：The no-shows have to be considered when deciding the rate of overbooking.（确定超过接待能力的预定时必须考虑预定了房间却来不了的客人.）
Ⅱ、主动语态变为被动的几个特殊情况

① 有些动词在主动结构中,后面接不带to的不定式,但如果改为被动,则需把省略的to加上,这类动词有 watch, make, help, let和感官动词等,如：The boss made my grandfather work 10hours a day.改成My grandfather was made to work for 10 hours a day.
② 含有宾语从句的主动结构变为被动,通常用it作为被动结构的先行主语,从句放在句子后面/也可采用另一种形式,这类动词有：know, say, believe, find, think, report等
③ 是所有的主动句都可以变换成被动句,更不是所有的被动句都可以*变换成主动句.虽然语法原则上允许主动和被动句的互相转换,但有的句子转换后会变成不通顺或不地道的英语句子.因此,在某些题目里,这也成为判断应该用主动还是用被动的依据.
例：At 5:05 p.m. on Saturday 19th July , there was an accident at the junction of the Main Street and Panda Road when a boy was knocked down off his bicycle by a delivery van. The boy was sent to St. Maria Hospital where he was treated for shock and a broken arm.
在这段文章里,a boy was knocked down off his bicycle by a delivery van这句被动句强调出读到文章的人最关心的事故的受害者.The boy was sent to St. Maria Hospital这句话则说明了孩子被送到医院的事实,至于是由谁（某个过路人?或肇事司机?）送的不重要.he was treated for shock and a broken arm这句被动句无须说出treat这个动作的发出者,因为在医院,伤病员自然由医务人员处理,无须罗嗦.这样,这段文章就重点突出,条理清楚了.
④ 有些动词可以有两个宾语,在用于被动结构时,可以把主动结构中的一个宾语变为主语,另一个宾语仍然保留在谓语后面.多是把间接宾语变为主语.这样句子自然些.直接宾语变为主语时,间接宾语要变为某个介词的宾语,介词to可以省略.如His father left him this house.改为This house was left (to) him by his father.
⑤ 有些动词虽为及物,但宾语并非是动作承受者,不能转换,这些动词有have, hold（容纳）,suit, fit, lack, become（适合）contain, cost, last, mean, suffice（足够）等.
⑥ 当直接宾语为反身代词、相互代词或宾语前有指代主语的物主代词时不用被动,如I shook my head.我摇摇头.
⑦ 当宾语为同源宾语（与主句指同一人）,动名词,动词不定式或一个从句时不用被动.如John enjoyed seeing the fil,.
⑧ 在一些固定说法中,有些名词和动词结合的固定说法,不能改We Chinese always keep our word.
⑨ 某些从不及物动词转化来的及物动词,直接宾语在表示动作的方式或效果时,这些动词在意思上起状语的作用,没有被动The girl kissed her boyfriend good night=The girl said good night to her boyfriend by kissing him.(这个女孩说了晚安并且亲了他的男朋友）
⑩ 表地点\处所\组织\长度\大小\数量\程度\抽象名词的词做宾语时不用被动.
⑪ 某些“不及物动词+介词”短语,walk into, listen to, sleep in, agree with, shake hands with, belong to, take part in, keep up with不能用被动.
⑫ 某些词用主动表被动：sell, miss, build, grow, look, smell, taste, sound, feel等

一、 被动语态的用法：

1. 一般现在时的被动语态构成：is / am / are + 及物动词的过去分词
Our classroom is cleaned everyday.
I am asked to study hard.
Knives are used for cutting things.
2. 一般过去时的被动语态构成：was / were + 及物动词的过去分词
A new shop was built last year.
Dinosaur eggs were laid long long ago.
3. 现在完成时的被动语态构成：has / have + been + 及物动词的过去分词
This book has been translated into many languages.
Many man-made satellites have been sent up into space by many countries.
4. 一般将来时的被动语态构成：will+ be + 及物动词的过去分词
A new hospital will be built in our city.
Many more trees will be planted next year.
5. 含有情态动词的被动语态构成：情态动词+ be + 及物动词的过去分词
Young trees must be watered often.
Your mistakes should be corrected right now.
The door may be locked inside.
Your homework can be handed in tomorrow.
6. 现在进行时的被动语态构成：am / is / are + being + 及物动词的过去分词
Uncle Wang is mending my bike now.→
My bike is being repaired by Tom now.
They are planting trees over there. →
Trees are being planted over there by them.
7. 不定式的被动语态：to + be + 及物动词的过去分词
There are two books to be read. →
There are twenty more trees to be planted.


二、 怎样把主动语态改成被动语态?

把主动语态改为被动语态非常简单,可以遵循以下几个步骤：
1. 先找出谓语动词；
2. 再找出谓语动词后的宾语；
3. 把宾语用作被动语态中的主语；
4. 注意人称、时态和数的变化.
例：1. Bruce writes a letter every week. →A letter is written by Bruce every week.
2. Li Lei mended the broken bike this morning.→The broken bike was mended by Li Lei this morning.
3. He has written two novels so far.→Two novels have been written by him so far.
4. They will plant ten trees tomorrow.→Ten trees will be planted by them tomorrow.
5. Lucy is writing a letter now.→A letter is being written by Lucy now.
6. You must lock the door when you leave.→the door must be locked when you leave.

三、 使用被动语态应注意的几个问题：
1. 不及物动词无被动语态.
What will happen in 100 years.
The dinosaurs disappeared about 65 million years ago.
2. 有些动词用主动形式表示被动意义.
This pen writes well.
This new book sells well.
3. 感官动词或使役动词使用省略to的动词不定式,主动语态中不带to ,但变为被动语态时,须加上to .
例：make somebody do something→somebody+ be +made to do something
see somebody do something→somebody +be +seen to do something
A girl saw my wallet drop when she passed by.→My wallet was seen to drop by a girl when she passed by.
The boss made the little boy do heavy work.→The little boy was made to do heavy work by the boss.
4. 如果是接双宾语的动词改为被动语态时,直接宾语(物)作主语,那么动词后要用介词,这个介词是由与其搭配的动词决定.
He gave me a book.→A book was given to me by him.
He showed me a ticket.→A ticket was shown to me by him.
My father bought me a new bike. →A new bike was bought for me by my father.
5. 一些动词短语用于被动语态时,动词短语应当看作一个整体,而不能丢掉其中的介词或副词.
We can’t laugh him. →He can’t be laugh by us.
He listens to the radio every day. →The radio is listened to by him every day.
The nurse is taking care of the sick man. →The sick man is being taken care of by the nurse.