Patterns containing V-tr + O (a transitive verb followed by its object) can be turned into the passive. As there are four kernel patterns of this kind, there are four different forms of passive voice:
SVO produces passive forms where O becomes S, V becomes V-P (verb, passive) and S may become Ag (agent).
(2c) The earthquake / destroyed / the town
(2cP) The town / was destroyed / by the earthquake
The agent is often omitted in generalisations and de-personalisations:
(2d) This phenomenon is not well understood.
SVOO, instead, produces a passive form where the indirect object of the active form becomes the subject, while the object remains at the end as retained object of verb (N-rov):
(5a) They told me the news
N-sv V-P N-rov
(5aP) I / was told / the news (/ by them)
When SVOC is made passive, the object complement remains at the end of the sentence as a retained object complement (roc), either nominal or adjectival:
N-sv V-P N-roc (Ag)
(6aP) Mr Lambe / was elected / chairman (/ by the directors)
N-sv V-P Aj-roc (Ag)
(6bP) He / was considered / honest and trustworthy (/ by everybody)
(6c) The surfaces of the wafers / are polished / flat
SVOA patterns behave similarly, retaining A at the end of the passive sentence.
(7c) The receptionist / showed / the new guest / into the waiting-room
(7cP) The new guest / was shown / into the waiting-room
(7dP) Transistors / are used / in a wide array of electronic equipment
All these remarks are valid from the point of view of syntax, but not all V-tr allow a passive form: I have three daughters cannot be turned into *Three daughters are had by me (some linguists consider to have a linking verb or class it in a special group, different from transitive verbs proper).
Several verbs can be turned into the passive when the are used transitively, not when they function as linking verbs. Compare:
(9) The butcher weighed the slice of beef
(9P) The slice of beef was weighed by the butcher
whereas
(9a) The butcher weighed over ninety kilos
does not allow
(9aP) *Over ninety kilos were weighed by the butcher.
In fact, in (9a) over 90 kilos is not an object but a subject complement in a SVC pattern.
The use of the passive voice may be barred by coherence in discourse. Given the question:
(10) What did the Romans do in the first century AD?
the following answer is not acceptable
(10a) *Britain was occupied by the Romans in the first century AD.
because the question posits the Romans as the theme of the answer, which then is to be:
(10b) The Romans occupied Britain in the first century AD.
This shows how, except on a purely formal level, it is not true that a passive sentence is equivalent to the corresponding active sentence. There are cases where the passive voice is not barred but still highly improbable; for instance, in
(11) I need money
the communicative focus is on what is wanted: money and not, suppose, help or a pen. If a sentence like
(11a) Money is needed by me
ever occurs, then by me is emphasised and implicitly contrasted with "not by X" — where X is any person who was mistakenly thought to be in need of money. In other words, (11a) implies that the need for money is a piece of information that is already shared, while the new information is about the person in need: "It's me the one who needs money, not somebody else". This situation is certainly less likely to occur in everyday life that the other (I need...).
The following case is only partly similar:
(12) John forgot to post the letter
(12P) *To post the letter was forgotten by John
The passive form is not acceptable; if focus on the subject is desired, then a cleft sentence is to be used:
(12a) It was John who forgot to post the letter.
Cleft and pseudo-cleft sentences will be discussed below (§ 2.1).