To be + past participle

The door is always locked

The letter was posted yesterday

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

The secretary typed the report

We wash the dishes every day

Darren played the game

We use a passive verb to say what happens to the subject. Who or what causes the action is often unknown:

The report was typed (By who? We don't know - it is unimportant)

The dishes are washed every day

However, if we want to say who does or what causes the action, we use "by…".

The report was typed by the secretary

The game was played by Darren