There are several cases where the passive voice is preferred over the active voice in English. Most of them are instances where the agent ("who" or "what" does the action, the subject of the verb) is either unknown, or unclear, or insignificant/not relevant in the context. For example, general words such as "people" or "everybody" are often left out when they are self-evident and are not necessary for meaning. Thus, you would say "My wallet was stolen from me a few days ago" and not something in the likes of "A thief stole my wallet from me a few days ago" (it is unnecessary to specify that "a thief" stole from you!).

Similarly, when the agent is unknown, English speakers also prefer to use the passive voice.

Example: "A UFO has allegedly been seen hovering in the sky of London last night". Seen by whom? That is precisely the doubtful element here… In this case, in English, you would neither use the (rare) pronoun "one" nor any other general word such as "someone" or "some people" or "a person" – that would be felt as a gallicism (a French calque).

In a nutshell: if the agent is either unknown, ambiguous, or insignificant/irrelevant, use the passive voice in English, and avoid the general pronoun "one" which is an old-fashioned, literary word used mostly in some fixed expressions. Remember that, in a sentence in the passive voice, the emphasis is placed on the object. In other words: from the classic pattern subject-verb-object, the conventional word order is reversed and the object becomes the grammatical subject.
Example: "Mosquitos bit me last night" (S=mosquitos/V=bit/O=me) will become "I was bitten by mosquitos last night" (S=I/V=was bitten/O=none). Here you see that the original object ("me") has become the subject ("I"), the verb has been turned to the passive voice (auxiliary BE or GET + PAST PARTICIPLE), and the subject has become a simple complement.

KEEP THIS IN MIND: The latter sentence (i.e. "I was bitten by mosquitos"), in the passive voice, is more genuine and idiomatic in English where human subjects are often preferred over animal or inanimate subjects. Thus, if you hesitate between:

  • "I was soaked all over by the rain" ;
  • "The rain soaked me all over" ;

choose the first one with a human subject (and a verb in the passive voice, then).