Passive (is done/was done) Study this example:
This house was built in 1935.
Was built is passive.
Compare active and passive: Somebody built this house in 1935. (active)
This house was built in 1935. (passive)

When we use an active verb, we say what the subject does:
* My grandfather was a builder.He built this house in 1935.
* It's a big company. It employs two hundred people.

When we use a passive verb, we say what happens to subject:
* This house is quite old. It was built in 1935.
Two hundred people are employed by the company.

When we use the passive, who or what causes the action is often unknonw or unimportant:
* A lot of money was stolen in the robbery. (somebody stole it, but we don't know who)
* Is this room cleaned every day? (does somebody clean it? - it's not important who)

If we want to say who does or what causes the action, we use by ...:
* This house was built by my grandfather.
* Two hundred people ar eemployed by the company.
The passive is be (is/was etc.) + past participle (done/cleaned/seen etc.):
(be) done, (be) cleaned, (be) damaged, (be) built, be (seen) etc.

Study the active and passive forms of the present simple and past simple:
Present simple
active: clean(s)/see(s) etc.
Somebody cleans this room every day.
This room is cleaned every day.

Past simple
active: cleaned/saw etc.
passive: was/were + cleaned/seen etc.
Somebody cleaned this room yesterday.
This room was cleaned yesterday.