This house was built in 1930.

'Was built' is passive. Compare active and passive:

Somebody built this house in 1930. (active)

This house was built in 1930. (passive)

We use an active verb to say what the subject does:

- My grandmother was a builder. He built this house in 1930.

- It it's a big company. It employs two hundred people.

We use a passive verb to say what happens to the subject:

- This house is quite old. It was built in 1930.

- Two hundred people are employed by the company.

When we use the passive, who or what causes the action is often unknown 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 are employed by the company.

Present simple

active: clean(s)/see(s) etc.

passive: am/is/are cleaned/seen etc.

Somebody cleans this room every day.

This room is cleaned every day.

- Many accidents are caused by careless driving.

- I'm not often invited to parties.

- How is this word pronounced?

Past simple

active: cleaned/saw etc.

passive: was/were cleaned/seen etc.

Somebody cleaned this room yesterday.

This room was cleaned yesterday.

- We were worken up by a loud noise during the night.

- 'Did you go to the party?' 'No, I wasn't invited.'

- How much money was stolen?

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