When we want to say that something was done, we use the verb to be and we put –ed at the end of a verb. This grammar is called the passive.
We use the passive when we don’t know who does an action.
There are some verbs which have their special form for the passive. There are 16 of these words for Level 2. These words are
broken, built, chosen, done, eaten, found, frozen, hidden, known, lost, made, paid, stolen, understood, woken up, written.
You can see more examples in the book The Little Prince.