Is using passive voice "bad form"?

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.

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.

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.

I agree with Peter. It can serve as a cohesion device for juggling new information (usually contained in the predicate of a sentence) and old information (usually put into the subjecct of a sentence). Passive can also be the expected style in certain genres (science).

In English departments in America, professors teach stupid things like: avoid the verb 'be'... never use the passive voice. I think MS Word has cravenly defaulted to the writers' memories of freshman English classes where they were tasked with writing lively personal essays.