语态是动词的一种形式，它用来表示句中主语同谓语动词之间的关系。
英语有两种语态， 主动语态和被动语态， 主动语态表示主语是谓语动词动作的执行者，被动语态表示主语是谓语动词动作的承受者。例如：
I have repaired the radio.
我修好了收音机。
The radio has been repaired.
收音机被修好了。
The students cleaned the classroom.
学生们打扫了教室。
The classroom was cleaned by the students.
教室被学生们打扫了。
被动语态的构成：
be + 过去分词
A building was damaged by the storm.
暴风雨毁坏了一座建筑物。
Our plate was made in China.
我们的盘子是中国生产的。
My bike was stolen.
我的自行车被盗了。
常用的被动语态的时态变化如下，以 ask 为例：
一般 进行 完成
现在I am asked, I am being asked, I have been asked
过去I was asked,I was being asked,I had been asked
将来I shall be asked, I shall have been asked
过去将来I shall be asked
被动语态的疑问句是把助动词提前到句首。
Has your TV set been repaired?
你的电视机修了吗？
Was the kite broken?
风筝破了吗？
Has the work been done?
工作结束了吗？
被动语态的否定式是在助动词后面加 not.
The letter has not been sent out.
信还没有发出去。
The little boy has not been found out.
小孩还没有找到。
The cap has not been mended yet.
帽子还没有补好。
Their money has not been sent to them.
他们的钱还没有送到他们手中。
被动语态的用法：
被动语态表示一种主语和谓语之间的关系，当主语是动作的承受者时， 就需要用到被动语态，汉语中所说的，“汽车被撞坏了”“钢笔被修好了” 等句子就是一种被动语态的句子，在汉语中常用“被”“由”等词来表示这种被动语态。在英语中则用动词的被动语态形式来表示。被动语态中还可用 by 短语加在句后表示被动语态中动作的发出者。也就是谓语动词动作的逻辑上的主语，被动语态常用在下列情况。
1) 不知道谁是谓语动词动作的执行者时，或者没有必要说出谁是执行者时。
The glass was broken last night.
玻璃昨天被打破了。
His bike has been stolen.
他的自行车被偷了。
He was asked to go there once more.
他被要求再去那儿一次。
The bag was put into the box.
袋子放在了箱子里。
2) 为了强调或突出动作的承受者时。
The plan has been sent to the headmaster.
计划已经送给校长了。
Kilinton was elected the President of U.S.A.
克林顿被选为美国总统。
Five people were hurt in the explosion.
五个人在这次爆炸中受伤。
A new building was set up in a short time.
新大楼短期就建成了。
3) 被动语态也可以说出动作的执行者，构成如下：
动作承受者 + be + 过去分词 + by + 动作执行者。
The picture was praised by everybody.
照片得到了大家的好评。
The classroom was cleaned by us.
教室被我们打扫过。
The bridge was built by workers from Shanxi.
这座桥是山西的工人建的。
The little boy was found by the police.
小孩被警察找到了。
有关被动语态的几个问题：
1) "be + 过去分词" 并非都是被动语态，系动词 be, feel, seem,look,等词后面的过去分词已转化为形容词，作表语用，表示某种状态。
I'm interested in mathematics.
我对数学感兴趣。
He seems unsatisfied with his work.
他看起来对他的工作不满意。
We are determined to catch up with the developed countries.
我们决心要赶上发达国家。
The song is called "Don't forget me".
歌曲的名字叫“勿忘我”。
某些动词用主动语态来表示被动含义。
The food tastes good.
食物很香。
It smells strange.
这有怪味。
The stone feels very cold.
石头摸起来很冷。
The woman's clothes sell well.
女装卖的快。
3) 一般情况下，及物动词才能构成被动语态，但有些不及物动词组成短语后，也可构成被动语态。
He was looked down upon because of his egoism.
他因自私而受人冷落。
The children were taken good care of in the the orphanage.
孩子们在孤儿院受到了良好的照顾。
参考资料：http://www.cnread.net/cnread1/yyxx/yyyf/015.htm