In one line: Passive = subject + be + past participle — used when the agent is unknown, unimportant, or de-emphasized.
Passive by Tense
1Present. is/are + PP.
2Past. was/were + PP.
3Perfect. has/have been + PP.
4Future. will be + PP.
Active → Passive
The teacher explains the rule.
The rule is explained.
Scientists discovered a planet.
A planet was discovered.
They will announce results.
Results will be announced.