1.Do you always send a letter to Mr Hu?
>Is a letter always sent (by you)to Mr Hu?
2.Do the workers build many buildings?
>Are many buildings built (by the workers)?
3.How many desks do they buy every term?
>How many desks were bought (by them every term?
方法步骤：
1、把主动句中的宾语找出来,变成被动句中的主语（代词需要变成主格）；
2、把主动句中的谓语动词变成被动式（原句的时态不变,句子的种类不变,原来是肯定句就用肯定句,疑问句就用疑问句,否定句就用否定句）；
3、把主动句中的主语变成by+主语的形式放在谓语被动结构的后面(代词要用宾格)；
4、如果主动句子中有修饰谓语的副词,要移到被动句子谓语动词结构中第一个助动词的后面.