Very often the subject of the action desribed by the verb is unknown, irrelevant. There are several possibilities to cope with this problem. The first solution is to use the passive voice.
A lot of money is spent for useless things.
In this case the subject of the sentence is the goal of the action described by the verb, but not the executor and there is no need to mention it.
The other possibility is to construct with an indefinite pronoun like "one".
One spend too much money in useless things.
The undefined subject is always a human being. If we talk for instance about dogs we name them: "Dogs interact intensively with human beings." Therefore we can construct as well with any noun or pronoun that refers to the whole species. The personal pronoun we for instance can refer to all human beings. Therefore we can say as well.
We spend too much money in useless things.
However in some contexts, for instance if a couple is talking about their financial situation, we is only a limited group. Another solution is the noun people.
People spend too much money in useless things.
However there are differences concerning the abstract subject. These sentences doesn't have the same meaning.
1) The apples are sold.
2) One sells apples.
3) People sell apples.
In this case only 1) make sense. Apples can only be sold by people who produce apples or trade with apples. 2) suggests that everybody sells apples, what is obviously not the case. And 3) is still worse. It suggests that people have the habit to sell apples. I other contexts a substitution is possible.
1) Too much water is consumed.
2) One consumes too much water.
3) People consume too much water.
However there is an another structure: The book sells well. This construction ressembles a little bit to the portuguese construction we will see next. althougt the reflexive pronoun is missing.
In Portuguese an undefined or irrelevant executer can be described with two different structures that resembles each other, although they are completely different from a grammatical point of view. The first construction is the se passivo and the second is the se impessoal.
Houses are sold. (houses for sale)
The photos are developed in less than two hours.
A literal translation of "Se vendem casas" would be "The houses sell themselves". The meaning of the sentence is therefore that the houses sell themselves what they actually don't do. They are sold by someone else. The same is true for the second sentence. The photos don't develop themselves, they are developed by someone else. This construction is an alternative two the normal passive voice, see passive voice, which is rarely used in roman languages. As well as the normal passive voice this construction is always possible, if the active sentence would have a direct object, because the direct object of the active sentence becomes the subject of the passive sentence, in other words, it works with all transitive verbs, with all verbs that have or can have a direct object, (to see => I see the house, to sell => I sell the house, to buy => I buy the house etc.).
The construction above looks very similar to the construction below, however this construction is completely different.
One works little here.
The police is well respected here.
This se is not a reflexive pronoun, it is an indefinite pronoun and the sentence is not in passive voice. The se is the subject and executor of the action described by the verb and trabalho is the indirect object. Concerning the second sentence "Aqui se respeita muito à policia" it is true that the sentence is translated to English with the passive voice and one may assume that this is possible as well in Portuguese. However this is not the case. The police is not a directe object but a prepositional object, is added to verb not directly, but through a preposition. Intransitive verbs, verbs that doesn't have and can't have a direct object, to go => ~I go the house, to swimm => ~ I swim the swimming pool, can never form a passive sentence. A sentence like "The swimmingpool is swum" is pure nonsense. ("The times are gone" is possible, but then gone has a different meaning.)
In Spanish and Italian it is an error to confuse this two construction. In Portuguese, most of all in Brasil, there is tendency to accept a construction with a se passivo although it is a se impessoal. A discussion can be avoided by doing it in a way that would be grammatically logical.
wrong: Se vende casas.
right: Se vendem casas.
Vender, to sell, is a transitive verb and in an active sentence, One sells houses, houses is the direct object. This sentence can be and must, at least in Spanish and Italian, put in passive voice and therefore the subject, third person plural, has to agree with the verb. This is an error very often made even by native speakers. As already said, in Brasil there is a tendency to accept sentences like "Se vende casas". If an "error" is committed so often, it is not an error anymore.