From a purely grammatical point of view passive voice means that the subject of the sentence is not the executor of the action described by the verb but the goal of this action.
1) He repaires the care.
2) The car is repaired by him.
In 1) he is the subject of the sentence an executes the action described by the verb. In 2) the car is the subject of the sentence but doesn't execute the action. The car is the goal of this action. Some people have problems to understand what is the subject of a sentence. The subject of the sentence governs the verb. That means that the subject agrees in gender (not in English, but in general) and number with the verb. If we put the sentences above in plural we get.
They repare the cars.
The cars are repaired by them.
We see that the verb changes if we set the subject into the plural form. Instead of repairs we get repair and instead of is we get are.
However that doesn't explain why we need a passive form. In the sentence above the active voice is as nice as the passive voice but much simpler. There is no need for the passive voice. We need the passive voice if we know the goal of an action, but not the executor of this action or if we are not interesting in mentioning it. This can happen for different reasons. Lets have a look at the following sentences.
The executor is definitely unknown: My wallet was stolen.
The executor is known, but only vaguely defined: The oranges are peeled and strewed with cinnamon.
The executor is irrelevant: To much plastic waste is produced in Europe.
There is an interest to veil the executor: Due to a delivery delay the merchandise couldn't be delivered already.
Case 1. is obvious. If he knew the person who has stolen his wallet, his problem could be eventually resolved, in any case there would be a bigger chance to get it back.
Case 2. is a little bit more complicated. The executors of the action is more or less known, these are the people who are reading the receipt in a cook book and try to make this marvellous orange cake. Obviously there is a more polite way to say that, for instance "You peel the oranges and strew them with cinnamon". However some people, for instance bureaucracies, feels that it offends their dignity to be more polite. Bureaucracies like the passive voice: Your tax return is to be filled on a different tax form. In this case there is an other advantage. You don't know who is actually responsible in case they make an error.
Case 3: There are a lot of executors, but that doesn't matter. Private households, the industry, the mail-order selling, the grossery stores etc. The only alternative to not mentioning them at all would be to mention them all.
Case 4 means in reality: We have forgotten to deliver the merchandise because our organisation is completely chaotic. However there is no one really responsible for that. An unknown subject is responsible for our error, there is very little we can do.
Furthermore the passive voice can be used in other contexts as well. It can be used for instance to express an order.
It is to be done this way!
An alternative for the passive voice is a construction with an indefinite pronoun like one or with a noun that embraces an unknown group like people.
A lot have been talked about it.
One has talked a lot about it.
People have talked a lot about it.
However this author is not really sure that these construction really are exchangeable in any context.
a) The issue has been discussed.
b) One discusses about the issue.
c) People discuss about the issue.
a) refers more to a concrete context. There is a problem in a concrete company for instance and employees discuss this issue. b) Is more general, at least that is what this author believes. If there is a problem that concerns the whole nation people discuss about it. The same is true for c. The boundaries may be fluid, but this author would say that there is a difference.
Lets have a look at this example.
c) The cherries are picked up.
d) One picks up cherries.
This author would say that d) means that it is a common custom to pick up cherries, as common as taking a bath, that it is done by everybody on regular basis. However it is clear that this can't be true. Only people with access to a cherry tree can pick up cherries and there are very few people who have access to a cherry tree. c) is more restrictive. Determinated cherries in a concrete context are picked up. This is at least the "feeling" of the author concerning this issue.
Many languages, for instance Portuguese and Spanish but not English, Italian and French have two version of the passive voice: a processual passive and a statal passive. A processual passive describes a process, an ongoing action, meanwhile the statal passive describes the result of this process and a state. This distinction is not made in English, however the difference is easy to understand. If we form the passive voice with a continuous form we describe the action as ongoing, because the continuous form always describes an action as ongoing. (We abstract from the fact that no native English speaker would say that. It is just an illustration of the principle.)
1) processual passive:
The dinner was being prepared when we arrived.
2) statal passive:
The dinner was prepared when we arrived.
In the first case the dinner was not ready when they arrived, someone was preparing it. In the second case the dinner was already prepared when they arrived. In the first case they risk to be asked for help, in the second case they can just sit down.
The passive voice is rarely used in Portuguese, because there is an alternative, the se passivo and the se impessoal. We will discuss that in the next chapter, see 17.1.
The processual passive in Portuguese is formed with auxiliary verb ser and the statal passive with the auxiliary verb estar. Apart from that it is the same construction as in English. The table below only shows the third person singular in indicative mood. We translate with an English tense with the same structure. A compound tense in Portuguese with a compound tense in English and a simple tense in Portuguese with a simple tense in English. However that doesn't mean at all, not at all, that they are used in the same context. Most of all the pretérito perfeito simples can be used in a context where we use, depending on the context, the past tense or the present participle, see 9.2. We do that for didactical reasons, but that doesn't mean that the preterito perfeito composto corresponds semantically to the prestent perfect.
actually a translation with a continuous form would be more correct
The apple is eaten.
The apple was eaten.
The apple was eaten.
The apple has been eaten.
The apple had been eaten.
The apple will be eaten.
The apple will have been eaten.
The apple would be eaten.
The apple would have been eaten.
In grammar books it is often underlined that the past participle has to agree in gender and number with the subject of the sentence.
femininum / Singular
A maçã é comida.
femininum / Plural:
As maçãs são comidas.
maskulinum / Singular:
O livro é lido.
maskulinum / Plural:
Os livros são lidos.
However it is questionable whether this is really a special rule. Ser and estar are copula verbs they assign a characteristic to the subject. The sentences "The book is red", "The book is a lie", "The book is written" have all the same structures and the predicative, the characteristic assigned to the subject by the copula verb has to agree in gender and number with the subject. Therefore there is no special rule here.
As maçãs são vermelhas. <=> The apples are red.
As maçãs são comidas.>=> The apples are eaten.
The passive voice is not the result of the construction as a whole. The passive character steems from the past participle alone, which has always a passive meaning. If the past participle is used as an adjective, it has a passive meaning as well: The repaired car was as good as new. The car repaired corresponds to the car that has been repaired.
The same thing is true for estar. The difference between the passive voice formed with ser and the passive voice formed with estar is that the passive voice formed with ser describes a process, the passive voice formed with estar a state, the result of a process, see above. Concerning the tenses used in the English translation see above. If two tenses resembles each other in the way they are formed, for instance if they are both formed with the auxiliary verb to have (ter) in present tense and the past participle, this is true for the present perfect and the pretérito perfeito simples, there is no guarantee that they are used in the same context. We translate in English with a tense that is formed the same way in Portuguese, but that doesn't mean that they are used in the same context. In other words: The English translations are wrong.