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)

Active:	Peter	builds	a house.
Passive:	A house	is built	by Peter.

 

Examples

Active	Peter	builds	a house.	Simple Present
Passive:	A house	is built	by Peter.

 

Active:	Peter	built	a house.	Simple Past
Passive:	A house	was built	by Peter.

 

Active:	Peter	has built	a house.	Present Perfect
Passive:	A house	has been built	by Peter.

 

Active:	Peter	will build	a house.	will-future
Passive:	A house	will be built	by Peter.

 

Active:	Peter	can build	a house.	Modals
Passive:	A house	can be built	by Peter.