被动语态的基本时态变化
被动语态通常为十种时态的被动形式, 被动语态由be+过去分词构成，be随时态的变化而变化。以do为例，各种时态的被动语态形式为：
1) am/is/are +done (过去分词) 一般现在时
例Visitors are requested not totouch the exhibits.
2) has /have been done 现在完成时
例All the preparations for thetask have been completed, and we're ready to start.
3) am/is /are being done 现在进行时
例A new cinema is being builthere.
4) was/were done 一般过去时
例I was given ten minutes todecide whether I should reject the offer.
5) had been done 过去完成时
例 By the end of last year,another new gymnasium had been completed in Beijing.
6) was/were being done 过去进行时
例A meeting was being held when Iwas there.
7) shall/will be done 一般将来时
例Hundreds of jobs will be lostif the factory closes.
8) should/would be done 过去将来时
例The news would be sent to thesoldier's mother as soon as it arrived.
9) shall/will have been done 将来完成时(少用)
例The project will have beencompleted before July.