If you learn grammar without asking why, you will find it very hard going.
That is tick-box learning: "right, I know the present simple", "right, I know the passive".
But genuinely: why would anyone use the passive? Why not the active? In what situation? Because it sounds impressive, or to reach band 8?
What the passive is actually for
Understanding and Using English Grammar puts it clearly: you use the passive when the agent, the person performing the action, is either unknown or unimportant.
Two examples
The door is broken. You have no idea who broke it. Rather than someone has broken the door, you say:
The door is broken.
Because nobody knows, and nobody needs to.
The lights were left on overnight. Rather than:
Someone left the lights on yesterday.
you say:
The lights were left on yesterday.
The first sounds like an accusation; it points at a person. The second points at the event, and reads as far more objective.
Why this matters
Once you understand that the passive exists to move the focus away from whoever did it, you no longer need to memorise a formula. You will know when to reach for it, because you know what it does.
That is the difference between learning a rule and understanding a tool.