In active sentences the subject is the person / thing that does the action. The subject comes at the start of the sentence. Most English is made up of active sentences. Take a look at this example using Paul as the subject and book as the object:
'Paul wrote a book.'
In passive sentences the focus of the sentence is the 'recieving' object of an action and it comes at the start of the sentence; therefore, the object and the subject change positions in the sentence:
'The book was written by Paul.'
To make a passive sentence you need to start with object first and then use the be form of the verb (depending on the tense) and then add a past participle:
object + be + past participle
Active: 'The gardener grows flowers.'
Passive: 'Flowers are grown by the gardener.'
Active: 'My grandfather planted that tree.'
Passive: 'That tree was planted by my grandfather.'
Active: 'The dog is chasing the sheep.'
Passive: 'The sheep are being chased by the dog.'
Now choose the correct word to make passive sentences: