這個是動詞的ED形式表被動的含義,be動詞是可以被省略的,寫成three is added to seven 也是正確的.
被動語態(tài)
英語的語態(tài)是通過動詞形式的變化表現(xiàn)出來的.英語中有兩種語態(tài)：主動語態(tài)和被動語態(tài).
主動語態(tài)表示主語是動作的執(zhí)行者.
例如：Many people speak Chinese.
謂語：speak的動作是由主語many people來執(zhí)行的.
被動語態(tài)表示主語是動作的承受者,即行為動作的對象.例如：Chinese is spoken by many people. 英語English是動詞speak的承受者.
1. 定義：被動語態(tài),即不知道動作執(zhí)行者或強調(diào)動作承受者的一種語態(tài).例如中文常說：我被他打,這就是一種被動.但有時由于句子結(jié)構(gòu)上的需要也要用被動,例如It is not unusual for workers in that region to be paid more than a month it 在句中作形式主語.而不定式to be paid more than a month是句子的邏輯主語.結(jié)合選項全句的意思是：“那個地方的工人一個多月后才得到工資是常有的事”.
2. 構(gòu)成：be+past participle(過去分詞)(簡稱P.P.)（+by+動作執(zhí)行者）（過去分詞的概念見上課）
3. 當(dāng)主動語態(tài)要被改成被動的時候,我們把原句的賓語提前,作為改句的主語,主語后置,作為賓語.因此有一點要注意,不及物動詞由于不加賓語,沒有被動形式,但不及物動詞如果與某些介詞構(gòu)成介詞短語,可以用被動.例如The fire had been put out before the fireman arrived. Put是不即物動詞,但put out是及物動詞
4. 應(yīng)用到各種時態(tài)和句型,如下：
① 一般式（一般現(xiàn)在,一般過去,一般將來）：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：“環(huán)境一旦遭到破壞,需要多年時間才能恢復(fù)過來.”do作為及物動詞有“引起,產(chǎn)生”的含義,do damage的意思是“造成破壞”.主語damage是及物動詞do的動作對象,謂語應(yīng)當(dāng)用被動語態(tài).同樣的,還有,I will mend the machine.相當(dāng)于The machine will be mended （by me）.
②進行時（現(xiàn)在進行、過去進行、將來進行）：be+being+P.P. ie.The classroom is being cleaned.
③ 完成時（現(xiàn)在完成、過去完成、將來完成）：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 其他時態(tài) 依詞類推,可得到結(jié)果.
⑤ 情態(tài)動詞的被動語態(tài)：主語 + 情態(tài)動詞 + be動詞 + 動詞過去分詞,例如Your teeth must be brushed.
⑥ 不定式的被動語態(tài)：to be done例：The no-shows have to be considered when deciding the rate of overbooking.（確定超過接待能力的預(yù)定時必須考慮預(yù)定了房間卻來不了的客人.）
Ⅱ、主動語態(tài)變?yōu)楸粍拥膸讉€特殊情況

① 有些動詞在主動結(jié)構(gòu)中,后面接不帶to的不定式,但如果改為被動,則需把省略的to加上,這類動詞有 watch, make, help, let和感官動詞等,如：The boss made my grandfather work 10hours a day.改成My grandfather was made to work for 10 hours a day.
② 含有賓語從句的主動結(jié)構(gòu)變?yōu)楸粍?通常用it作為被動結(jié)構(gòu)的先行主語,從句放在句子后面/也可采用另一種形式,這類動詞有：know, say, believe, find, think, report等
③ 是所有的主動句都可以變換成被動句,更不是所有的被動句都可以自由變換成主動句.雖然語法原則上允許主動和被動句的互相轉(zhuǎn)換,但有的句子轉(zhuǎn)換后會變成不通順或不地道的英語句子.因此,在某些題目里,這也成為判斷應(yīng)該用主動還是用被動的依據(jù).
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.
在這段文章里,a boy was knocked down off his bicycle by a delivery van這句被動句強調(diào)出讀到文章的人最關(guān)心的事故的受害者.The boy was sent to St. Maria Hospital這句話則說明了孩子被送到醫(yī)院的事實,至于是由誰（某個過路人?或肇事司機?）送的不重要.he was treated for shock and a broken arm這句被動句無須說出treat這個動作的發(fā)出者,因為在醫(yī)院,傷病員自然由醫(yī)務(wù)人員處理,無須羅嗦.這樣,這段文章就重點突出,條理清楚了.
④ 有些動詞可以有兩個賓語,在用于被動結(jié)構(gòu)時,可以把主動結(jié)構(gòu)中的一個賓語變?yōu)橹髡Z,另一個賓語仍然保留在謂語后面.多是把間接賓語變?yōu)橹髡Z.這樣句子自然些.直接賓語變?yōu)橹髡Z時,間接賓語要變?yōu)槟硞€介詞的賓語,介詞to可以省略.如His father left him this house.改為This house was left (to) him by his father.
⑤ 有些動詞雖為及物,但賓語并非是動作承受者,不能轉(zhuǎn)換,這些動詞有have, hold（容納）,suit, fit, lack, become（適合）contain, cost, last, mean, suffice（足夠）等.
⑥ 當(dāng)直接賓語為反身代詞、相互代詞或賓語前有指代主語的物主代詞時不用被動,如I shook my head.我搖搖頭.
⑦ 當(dāng)賓語為同源賓語（與主句指同一人）,動名詞,動詞不定式或一個從句時不用被動.如John enjoyed seeing the fil,.
⑧ 在一些固定說法中,有些名詞和動詞結(jié)合的固定說法,不能改We Chinese always keep our word.
⑨ 某些從不及物動詞轉(zhuǎn)化來的及物動詞,直接賓語在表示動作的方式或效果時,這些動詞在意思上起狀語的作用,沒有被動The girl kissed her boyfriend good night=The girl said good night to her boyfriend by kissing him.(這個女孩說了晚安并且親了他的男朋友）
⑩ 表地點\處所\組織\長度\大小\數(shù)量\程度\抽象名詞的詞做賓語時不用被動.
⑪ 某些“不及物動詞+介詞”短語,walk into, listen to, sleep in, agree with, shake hands with, belong to, take part in, keep up with不能用被動.
⑫ 某些詞用主動表被動：sell, miss, build, grow, look, smell, taste, sound, feel等

一、 被動語態(tài)的用法：

1. 一般現(xiàn)在時的被動語態(tài)構(gòu)成：is / am / are + 及物動詞的過去分詞
Our classroom is cleaned everyday.
I am asked to study hard.
Knives are used for cutting things.
2. 一般過去時的被動語態(tài)構(gòu)成：was / were + 及物動詞的過去分詞
A new shop was built last year.
Dinosaur eggs were laid long long ago.
3. 現(xiàn)在完成時的被動語態(tài)構(gòu)成：has / have + been + 及物動詞的過去分詞
This book has been translated into many languages.
Many man-made satellites have been sent up into space by many countries.
4. 一般將來時的被動語態(tài)構(gòu)成：will+ be + 及物動詞的過去分詞
A new hospital will be built in our city.
Many more trees will be planted next year.
5. 含有情態(tài)動詞的被動語態(tài)構(gòu)成：情態(tài)動詞+ be + 及物動詞的過去分詞
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. 現(xiàn)在進行時的被動語態(tài)構(gòu)成：am / is / are + being + 及物動詞的過去分詞
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. 不定式的被動語態(tài)：to + be + 及物動詞的過去分詞
There are two books to be read. →
There are twenty more trees to be planted.


二、 怎樣把主動語態(tài)改成被動語態(tài)?

把主動語態(tài)改為被動語態(tài)非常簡單,可以遵循以下幾個步驟：
1. 先找出謂語動詞；
2. 再找出謂語動詞后的賓語；
3. 把賓語用作被動語態(tài)中的主語；
4. 注意人稱、時態(tài)和數(shù)的變化.
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、 使用被動語態(tài)應(yīng)注意的幾個問題：
1. 不及物動詞無被動語態(tài).
What will happen in 100 years.
The dinosaurs disappeared about 65 million years ago.
2. 有些動詞用主動形式表示被動意義.
This pen writes well.
This new book sells well.
3. 感官動詞或使役動詞使用省略to的動詞不定式,主動語態(tài)中不帶to ,但變?yōu)楸粍诱Z態(tài)時,須加上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. 如果是接雙賓語的動詞改為被動語態(tài)時,直接賓語(物)作主語,那么動詞后要用介詞,這個介詞是由與其搭配的動詞決定.
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. 一些動詞短語用于被動語態(tài)時,動詞短語應(yīng)當(dāng)看作一個整體,而不能丟掉其中的介詞或副詞.
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.