Impersonal writing is very important in academic English and the passive voice is an important tool to achieve an impersonal style. Our Writing Pages explain why and when it is used. Here we provide links to pages that help you to revise how the passive is built and when it is used in everyday English.

The BBC Ask about English explains when and why the passive is used. It also offers a very clear table to check the correct forms in different tenses.
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv65.shtml

Monash University’s comprehensive tutorial leads you from recognition of passive forms to their use. Most of the examples are from academic texts.http://www.monash.edu.au/lls/llonline/grammar/passive/index.xml

The English Club offers a comprehensive list of passive forms and explains how they are formed in different tenses. There is also a quiz that asks you to choose between active or passive forms, but little explanation of the way the passive voice is used. http://www.englishclub.com/grammar/verbs-voice.htm

Change sentences into the passive voice in this quiz. http://esl.about.com/library/quiz/blgrquiz_passive1.htm

Choose between active and passive voice in these gap-filling exercises.
http://www.smic.be/smic5022/passive1.htm

Although grammatically possible, some forms of the passive are not used in English – this page explains which forms are unusual and why.
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv119.shtml