There are two styles of passive. We can have long passives which mention the agent:
Or we can have short passives that don't tell us who the agent is at all:
Whether you use an active or passive sentence will depend primarily on two factors. The first is which entities in your sentence have already been mentioned and which haven't. As a rule of thumb the best place for new information is at the end of the sentence. It is the end of sentence which carries most focus. Putting new information at the beginning of a sentence puts undue stress on your reader and makes sentences difficult to cognitively process. This is exacerbated when the first phrase is an indefinite noun phrase. Consider the following example:
- a. I've been studying the Mona Lisa. Leonardo Da Vinci painted the Mona Lisa.
This pair of sentences is awkward to say the least. It is certainly an example of poor style. What is the problem? Well, the major problem is that it uses the an active clause. The writer has ill-advisedly gone out of their way to avoid a passive. The result is that the discourse-old Mona Lisa is occurring at the end of the second sentence with all the grace of a limping warthog, whilst the revelatory information, the star attraction in the sentence, Leonardo Da Vinci appears at the beginning. Because Da Vinci occurs at the beginning it gives the reader the impression that something more informative,
more exciting and more revelatory is going to occur further on in the utterance. Hearts arrested, breath abated we wait in anticipation ... to find the boring old bloody Mona Lisa sitting at the end there. Now that's just rude. The writer had no reason to do that to us and then let us down so badly. This pair of sentences break some basic conventions of how speakers (and writers) package information. Here's what the writer should have done:
- b. I've been studying the Mona Lisa. The Mona Lisa was painted by Leonardo Da Vinci.
The reason the passive sentence works well is that the Mona Lisa links back to the first sentence, whilst Da Vinci, which is new to the discourse, takes the prominent position at the end.
The second factor which will affect whether you want to use an active or passive construction is which entities in your sentence you wish to specify and which you don't. There are two types of passive sentence, long passives and short passives. Long passives are usually used for the purposes given above. Short passives are often used when:
- we don't want to mention the agent for political or social reasons
- we don't know who the agent is
- the agent is obvious and therefore redundant
- the agent is irrelevant to our conversation
- our knowledge of the agent is so vague that mentioning them is redundant
For any of the reasons above the fist sentence here is probably more effective than the second:
- I've been shot!
- Somebody's shot me!
If, however, the agent is someone important or interesting in the context of your text, then you will prefer an active clause over a short passive:
- The world cup has been won!
- Cameroon have won the world cup!
So, the upshot of all of this is you should just choose the active or passive according to your needs. If there is one rule of thumb to stick to, it is to put new information at the end of your sentence and old information at the beginning.