Theory:

In a normal sentence in English, there is a subject, a verb and the verb has an object.

David is eating an apple.

Here, David is the subject, eating is the verb, and an apple is the object. This is called the active form.

The position of these elements can be changed to give the passive form, where the subject and the object of the verb change places.

An apple is being eaten by David.

Here, an apple becomes the subject, the verb changes form from IS EATING to IS BEING EATEN, and the object becomes BY DAVID.

The position of an element corresponds to its importance in the sentence.

If David is the subject, then David is the most important element in the sentence. If the positions are changed, the reason is to give another element more importance.

David cleans our windows.

Here we are more interested in who cleans the windows than in the windows, so if that person is David, then he is the most important element.

It is the answer to the question:

Who cleans your windows?

If we change the sentence to

Our windows are cleaned by david.

Then we are more interested in the windows than in David. This is an answer to the question

What is cleaned by David?

It is important to understand how the verb changes in this process:

cleans > is cleaned

is cleaning > is being cleaned

cleaned > was cleaned

was cleaning > was being cleaned

has cleaned > has been cleaned

must clean > must be cleaned

supposed to clean > supposed to be cleaned

Notice that:

Gerund verbs or verbs ending in ING use BEING + past participle.

He is CLEANING the car > The car IS BEING cleaned.

Modal verbs use BE + past participle

He SHOULD CLEAN the car > The car should BE cleaned.

Verbs in the infinitive use BE or TO BE + past participle.

He is supposed TO CLEAN the car > The car is suposed TO BE cleaned.

Notice also that all the passive forms use the past participle, CLEANED, and CLEANING become CLEANED.

Normally the passive is used when the person doing the action is not known:

The door was left open.