Study materials
Two sentences about the same event. The difference is where you start from.
Someone stole my bike.
My bike was stolen.
In the second sentence the bike has moved to the front and the thief has vanished entirely. That is what the passive does: it shifts attention from the doer to what happened.
What it is for
The passive puts first not the one who acted but the thing the action happened to. The doer need not be named at all.
That is convenient in three cases: when the doer is unknown, when the doer is beside the point, and when you deliberately prefer not to point at them.
My bike was stolen last night.
The road is repaired every spring.
The formula that never changes
The passive always consists of two parts: a form of be plus the third form of the verb. The tense and the person sit inside be, while the action itself stays unchanged.
Coffee is grown in Brazil.
The bridge was built in 1890.
These letters were sent yesterday.
The form of be agrees with the new subject, not the old one. That is why the third example takes were: there are several letters.
When by really is needed
The doer is added only when they genuinely add something to the meaning. Most passive sentences carry no by at all.
The novel was written by a teenager.
The window was broken.
If the doer is merely somebody, people or they, there is no point naming them: avoiding that is why the passive was chosen.
The mistakes everyone makes
The first is losing the form of be.
The bridge built in 1890. → The bridge was built in 1890.
The second is taking the second form instead of the third.
The letters were wrote in May. → The letters were written in May.
The third is agreeing be with the old subject.
These letters was sent yesterday. → These letters were sent yesterday.
All three are caught by one look at the sentence. Is a form of be there, and is it followed by the third form of the verb?
Try it right now
Take three sentences about what is being done in your town: the streets are swept, houses are built, roads are repaired.
Say them in English in the passive, without naming the doer once.