There are now many grammar-checking programs online; you can feed them a page of prose and they will return you a list of errors. No writer on Earth will ever use such a program, for it will signal grammatical errors where there is mere stylistic flair, blighting your text with red lines and making you feel insecure about the simplest of utterances.
One of these systems' many problems is that they are programmed to look for the passive voice. This is when you write, "The war was won," instead of, "We won the war." It involves switching the object and the subject. It's a completely normal and common English structure, yet grammar-checking programs, and many an unsophisticated copy editor, will automatically flag any use of it as undesirable. University professors will often tell their first-year students to avoid it in their essays. Why this disdain for one of the language's charming variations?
George Orwell has a lot to answer for here. His 1946 essay Politics and the English Language is almost universally quoted as a model of stylistic instruction. In it, Orwell complains about euphemistic, clichéd and obfuscating language – as used in political debates in Europe in 1946. It contains six famous guidelines for writing good English, including the avoidance of figures of speech you are used to seeing in print, the avoidance of long, foreign or jargon words, and "(iv) Never use the passive where you can use the active." No. 4 is now written in stone for the English teachers of the world.
But Orwell himself says, in the same essay, "I have not here been considering the literary use of language." His complaint is about political power, and it is specifically situated in 1946. Orwell's favourite targets are extremists, whether communist or conservative, who don't want to take responsibility for terrible bloodshed. In the environment of 1946, the passive voice could be a particularly smarmy avoidance of reality: Cities were bombed; civilians were killed. And it's still the favourite way for politicians to sidestep bad behaviour ("mistakes were made").
Writers of news reports for broadcast media are also best instructed to avoid the passive voice as much as possible as short and uncomplicated sentences are the most easily understandable when you're listening to something. (Spot the passive in that sentence. Is it wrong?) Furthermore, there is pressure on broadcast writers to put themselves in stories to make the stories make more exciting and more honest – that is, to write, "My cameraman and I heard three shots," rather than, "Three shots were heard." But news producers will tell you never to use subclauses or semi-colons, either. We don't all have to write like that all the time.
Sometimes the passive voice is the simplest way to express something. (When I'm talking about meaning in a sentence, for example, I might say, "it is understood that....")
Furthermore, it's just silly to try to eliminate any ancient part of the language by fiat. Any confident writer or speaker is able to find great elegance and fun in the most convoluted of sentences. I'm not going to tell Henry James to rewrite this: "The implements of the little feast had been disposed upon the lawn of an old English country house in what I should call the perfect middle of a splendid summer afternoon." A computerized grammar checker will flag "had been disposed." (And it will also tell you never to start a sentence with a conjunction. Like "and." Or to write sentence fragments like this.)
It's interesting that Orwell is also opposed to what he calls the "not -un" formation, as in phrases like, "In my opinion, it is not an unjustifiable assumption that ..." (instead of "I think"). This construction is technically called litotes; it's a form of deliberate understatement that is often used comically. "I am not unfamiliar with the effects of alcohol," one might say in a pub to general laughter. One wouldn't need to rail against this device in any contemporary essay because it belongs to a style of writing that is no longer popular; its very archaism makes it charming. Bear this in mind when reading Orwell's essay now.
I suspect, too, that had George Orwell ever encountered a style-checking computer program, he might had been tempted to add rule No. 7: "Never let a machine write for you."