Passive voice is used:
when we want to shift the importance
Example:
Jaka gave Dessy flowers
Dessy was given flowers by Jaka

when we don't know who did it or it is not important who did it
Example:
The murderer was finally arrested

to make the information sound more formal
Example:
Action must be taken immediately

To change a sentences from active into passive:
1. The object becomes the subject in the passive voice
2. The tense remains the same tense, but it is moved from the main verb to the verb 'be'
3. The verb is in past participle

Examples:
* Simple Present
ACTIVE (A) : George helps Bella
PASSIVE (P) : Bella is helped by George

* Present Continuous
A : George is helping Bella
P : Bella is being helped by George

* Present Perfect
A : George has helped Bella
P : Bella has been helped by George

* Simple Past
A : George helped Bella
P : Bella was helped by George

* Past Continuous
A : George was helping Bella
P : Bella was being helped by George

* Past Perfect
A : George had helped Bella
P : Bella had been helped by George

* Simple Future
A : George will help Bella
P : Bella will be helped by George

* Be going to
A : George is going to help Bella
P : Bella is going to be helped by George

* Future Perfect
A : George will have helped Bella
P : Bella will have been helped by George