In a recent ESL test, the question was whether the following sentence contained the passive voice: People should be concerned about how to find alternatives to fossil fuels.
The answer book says yes, and I see the logic to that when the definition of concern is to make some worried. However, can it also be argued that this sentence is active and concerned is an adjective?
I would tend to agree.
Past participles in English can be promoted to adjectives. And this can result in sentences that can be parsed in several different ways.
To distinguish you can ask "could I modify this word with adverbs of degree, like very?" and "could I form comparative or superlative forms with more or most?" If you can, that would suggest the word is an adjective. On the other hand you could ask "can I add a by someone phrase", to provide the subject of a corresponding active clause, or "can I rephrase in the active voice". Either of those would suggest that the word is a verb.
To some degree both adjective and verb are possible
People should be very concerned
People should be concerned by something about...
Something should concern people about ...
Looking at these, the first is completely acceptable. The second is somewhat odd (the about part doesn't fit) and the third doesn't really work at all.
In conclusion "be concerned" is best understood as an adjective, and not a past participle forming the passive voice.
I am going to argue for the opposite point of view from James K: yes, this should be considered a use of the passive voice.
The passive voice is characterized by the verb to be and the past participle of a verb. We have to be, with the modal should, and concern is definitely a verb, with the same meaning that it has here, of which concerned is the past participle. So grammatically, it's definitely in the passive voice.
The issue is slightly complicated by the fact that about isn't in how we usually construct passive sentences. If we make "A bear ate him" passive, it's "He was eaten by a bear"; if we make "The news hurt her" passive, it's "She was hurt by the news." But I think we need to consider about as kind of idiomatic with concerned and it is certainly possible to say "concerned by".
Lastly, I think (and I think this is where James K and I mostly agree) the difference is kind of academic in a lot of cases, including this one. You can always think of a passive sentence as being like "They possess the state of concerned-about-X", and participles as a way of adjective-izing verbs.
