"ce sont de vieilles maisons"
When I was doing timed practice, I got the phrase: "ce sont de vieilles maisons". Because it is plural, I would expect "des" instead of "de", but it was marked wrong. Could somebody explain to me why "de" is used instead of "des"?
Indeed "des" becomes "de" in front of an adjective but ONLY when there is "ce sont". A fairly common alternative is the use of "c'est" which takes "des" to indicate the plural. Notice that even with "c'est" there is no ambiguity whether it's a singular or a plural. In both cases the plural is obvious:
Ce sont de vielles maisons. C'est des vielles maisons. vs C'est une vieille maison.
However it's worth noting that "ce sont de" is falling into disuse, to say the least, because of its homophony with "ce sont deux" which may sometimes cause misunderstandings: I thought there were only TWO old houses.
Common use does not mean correctness and c'est des vieilles maisons is still improper French. This does not show a high level of education and it is really unwanted in writing.
"ce sont de" and "ce sont deux" are not homophones, since de is [duh] and deux is [deuh] (dö).
Improper grammar, spelling and pronunciation are actually common with the same people.
I only noticed a clear distinction between the sounds written <e> and <eu> from Québecers. I could say with much confidence that in France they are completely homophones for most speakers, except of course when <e> is deleted in speech, then you can obviously hear the difference.
In Belgium they replace the sound written <ui> with <oui> yet I'm sure any properly educated person wouldn't dare to look straight in their eyes and say "You must have a low level of education since the way you speak is utterly incorrect."
So indeed "c'est" is unwanted in formal writing because using "ce sont" shows humility while "c'est" doesn't. Plus in writing homophony isn't a problem at all. But otherwise it's perfectly fine to use it. I have plenty of university teachers (so very educated people) who use profusely "c'est des" and "c'est les" yet nobody ever complained or dared to "correct" the lecturer.
I understand your concern to teach the proper grammar, which is perfectly founded since you don't want the duolingo learners to be discriminated against when they get out there in the ruthless french society yet methinks this ruthlessness is overrated.
The French society can be ruthless, but not about its own language. 20% of children aged 12 can't read properly and people unable to write French properly is a majority.
My own views are unimportant, beyond the fact that street talk is not part of Duo's teaching mission.
Thank you both! This is a great discussion because it really answers the question very well and clearly, and I have learned something that I, as a non-native speaker, consider to be a subtle difference. :)