If a sentence is in the passive voice the subject of the sentence is not the executor of the action described by the verb, but the goal of that action.
The bread is baked.
In this case the bread is the subject of the sentence, but not the executor of the action, it is the goal of that action. The verb depends on the subject of the sentence. Let' s have a look at these sentences.
The bread is baked.
The breads are baked.
As you can see, the verb depends on the subject. If the subject is in singular, the verb is in singular. If the subject is in plural, the verb is in plural.
The only fact that in the passive voice the subject is not the executor of an action but the goal of that action wouldn't justify the existence of this grammatical construction because if that were the only reason, we could construct as well in active voice.
The bread is baked. <=> He bakes the bread.
So, what is the passive voice needed for? We need the passive voice if the executor of an action is unknown, irrelevant or should not be mentionned for some reason.
a) My purse has been stolen.
b) In Europe more wine is cultivated than what is consumed.
c) The apples are being peeled and then cooked half an hour in the oven.
In the case of a) the the executor of the action described by the verb is unknown, that' s obvious, because if he were known, the problem wouldn't be as big. In the case of b) the executor is known, all the winegrower of Europe, but this is irrelevant because it is obvious. Case c) is a little bit more ambiguous. The executor is not really unknown. The executors are all the people reading the cooking recipe, but he is not really known either.
The same function as the passive voice achieve some indefinite pronouns. In most languages, for instance in german and French, there is a preference for constructing with indefinite pronouns instead of using the passive voice. Spanish and Italian have an alternative to the passive voice as well, as we are going to see. In English dominates the use of the passive voice in circumstances like the one we mentioned before. The problem is, that the need to not mention the executor of an action exists as well if the passive voice is not possible because there is no direct object in the sentence in active voice and it is not possible therefore to construct a sentence in passive voice. All other languages but English have a clear solution for that problem. In English there is no system if the passive voice is not possible. How to translate depends in these circumstances from the context as we will see. A possible solution is to use the indefinite pronoun one.
a) Someone has stolen my purse.
b) One cultivates more wine in Europe that one consumes.
c) One peels the apples and then one bakes them in the oven.