Over at Language Log, Geoff Pullum writes about how the BBC doesn't know what a passive construction is.

That problem is hardly limited to journalists. English teachers frequently have the same problem. In this issue of Purdue OWL News [mistakenly dated 2007], an e-journal that answers writing questions, one of the OWL Tutors analyzes "is using" as a passive construction.

In the follow-up issue for the next week, the OWL news editor published a correction, but his explanation still leaves a lot to be desired.

He writes:

The passive voice appears in any sentence where the subject isn't doing the action of the verb. "Susan is using the computer" is an active construction in the present progressive tense (describing an action that is occurring). "The computer is being used by Susan" is the passive voice (not tense OR verb).

One minor problem is "present progressive tense." "Present" is a tense. The progressive is an aspect. More serious is the explanation of the passive.

This is better than the original assertion, and the example sentence to illustrate the passive voice is a real passive, but the rational is still insufficient.

The passive voice is a specific construction with a specific pattern of verbs: an auxilliary form of TO BE is followed by the past participle. It is true as well that the subject of the passive has the same semantic role as an object in the active equivalent, but that semantic role alone is not sufficient to define the passive.

Consider a senentence like the following:

(1) Barbara underwent surgery.

Barbara certainly isn't performing any action here. She's experiencing it. Or what about this one:

(2) The door opened.

Ok, perhaps you think that the door is performing an action here. But compare it to

(3) Bob opened the door.

In (2) and (3), the door plays the same semantic role--it's the thing acted upon. Yet none of these three are passive, and indeed, we could make (3) passive if we wanted:

(3a) The door was opened by Bob.

The underlying problem with this sort of explanation (apart from the evidence that even people whose business is teaching others writing have a shaky grasp of basic grammatical principles) stems from an over-reliance on the semantic definitions that schoolbook grammars peddle (a noun is a person place or thing, a verb describes an action, etc.).