The passive is used when the action is more important than the person or thing that performs the action. Sometimes we do not know who did the action (e.g. A lot of tea is grown on Chiri Mountain.).
ACTIVE: Mona cooked dinner.
PASSIVE: Dinner was cooked by Mona.
ACTIVE: Mona mailed the letter.
PASSIVE: The letter was mailed by Mona.
ACTIVE: An accident happened.
ACTIVE: I fell.
Only transitive verbs (verbs that are followed by an object) are used in the passive. It is not possible to use the verbs such as happen, sleep, come, and seem (intransitive verbs) in the passive.