Active or Passive sentences [duplicate]

Whenever I create a document in Microsoft Word, it complains about a lot of my sentences being in passive voice. But, when I read that sentence aloud, it sounds fine to me. I am not sure if it is just me and will a statement in passive voice sound strange to a native speaker?

So, my question is, is it considered bad form to use passive voice generally? Or in some specific cases like written communications only?

Edit: If it is ok to use passive voice, then why does MS-Word complain?

It's never bad form to use passive form. It's just that in speech, we tend to use a lot of this, but there's nothing wrong with using the passive form in writing, or in speech.

From the Passive Engineer:

Despite the admonitions of grammar checkers, the passive construction has a legitimate function. When you want to emphasize results, use the passive.

Note that it mentions grammar checkers, which I suppose is what you are getting.

Wikipedia states that:

Many language critics and language-usage manuals discourage use of the passive voice....This advice is not usually found in older guides, emerging only in the first half of the twentieth century

Also:

In 1926, in the authoritative A Dictionary of Modern English Usage (1926), Henry W. Fowler recommended against transforming active voice forms into passive voice forms, because doing so "sometimes leads to bad grammar, false idiom, or clumsiness

It's really just style, but nothing else to worry about.

That is quite a big question but the basics of when to use the passive run something like this:

In the following kind of sequence:

E.T. is a film about an alien and a boy. It was directed by Steven Spielberg in 1981. Its most memorable scene is the one where the boy and alien fly on a bicycle.

it sounds odd to say "Steven Spielberg directed it in 1981", because the focus of interest is the film E.T. rather than Spielberg. We might also imagine a sequence like this:

E.T. is a film about an alien and a boy. It was released in 1981. Its most memorable scene is the one where the boy and alien fly on a bicycle.

Here we don't even care who released it, we are only interested in the date.

Contrast this to

Steven Spielberg was born in 1942. As a boy he owned a movie camera. He directed his first movie, Jaws, in 1976. He also acted in "The Blues Brothers" as the Cook County Clerk.

In contrast to the above case, here it sounds odd to say "Jaws was directed by him in 1977" since the focus of the narrative is Spielberg rather than Jaws.

In neither case would changing passive to active or vice-versa create a grammatical mistake, though, this is more a matter of style.

As other posters have pointed out, there's nothing objectively wrong with the passive voice. It's a useful, grammatically correct feature of the English language.

However some people are prone to overuse the passive voice, which is why many sources of writing advice discourage its use. Unfortunately, this advice somehow transformed from "use the passive use sparingly" to "the passive voice is WRONG!" which is a rather silly extreme.

(But: I once had the eye-opening experience of editing three pages of writing entirely in the passive voice. Reading it was like slogging through molasses, but it took me a while to identify the passive voice as the issue. Overuse of the passive really is bad writing, even if certain English teachers and software programmers go too far in the other direction.)

The Writing Center, University of North Carolina at Chapel Hill published this very good, Creative Commons licensed write-up on what passive voice is, why it might be discouraged, and when it is "okay" to use it.

Here's the same page on the WayBack Machine, just in case the original breaks again.