4TH-PASSIVE VOICE- INTRO 2

Passive - Use
We only use the passive when we are interested in the object or when we do not know who caused the action.
Example: Appointments are required in such cases.
We can only form a passive sentence from an active sentence when there is an object in the active sentence.
Form
to be + past participle
How to form a passive sentence when an active sentence is given:- object of the "active" sentence becomes subject in the "passive" sentence- subject of the "active" sentence becomes "object" in the "passive" sentence" (or is left out)

YOU WILL FIND A USEFUL DIAGRAM IN THE NEXT LINK:

http://www.englisch-hilfen.de/en/grammar/passive.htm