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Passive is used in different cases in English, usually when we are more interested in action and its object rather than the subject (or agent). Observe the following phrases:

ACTIVE PASSIVE
Mary typed the letter on a Macintosh computer. The letter was typed on a Macintosh computer.

In the first example, we are clearly interested in the person who wrote the letter (Mary), as well as the person who used a Macintosh. In the second example, we are not more interested in the person who wrote the letter (the agent); we are only interested in the fact that it has been written with a Macintosh.

Formation of Passive
Each active form corresponds to an active form and vice versa. The passive is formed using the auxiliary to be and the past participle. The tablet below describes the steps to transform an active phrase into a passive phrase:

(1) Tomar una frase activa Someone opens the door
(2) Identificar el tiempo del verbo Present Simple
(3) Transformar el complemento objeto en sujeto The door…
(4) Agregar be en el mismo tiempo del verbo identificado en la frase activa The door is…
(5) Cambiar el verbo al past participle The door is opened.

Some examples of transformation also using the modal auxiliaries:

(1)Someone is opening the door (2)Someone opened the door (3)Someone was opening the door (4)Someone has opened the door (5)Someone should open the door (6)Someone must have opened the door
(1) The door is being opened (2) The door was opened (3) The door was being open (4) The door has been opened (5) The door should be opened (6) The door must have been opened

Verbs that can not be passive
The passive is formed by transforming an object complement into a subject. Therefore, intransitive verbs (verbs that do not need an object complement) can not be used in the passive form. Some examples of intransitive verbs are: arrive, sleep, die, walk, rain, snow and smile. Not even some state verbs (verbs that describe a state or condition, and that have no passive form) can be used in the passive form. Some examples are have (= have), belong to, resemble, suit and fit (= be of the right measure).

Common mistakes with the passive
The passive is sometimes confused with two other structures: the perfect and continuous forms. These are the differences between them:

(1) Pasivo: be + past participle (2) Perfect: have + past participle (3) Continuous: be + -ING
(1) The door is closed (2) Someone has closed the door (3) Someone is closing the door