"What kinds do you paint, old ones?" is bad English. It should be "What kind do you paint, old ones?" and this is not even accepted by duo.
I think “kind” singular should be accepted as common usage. But kind/type/sort can all be plural and should match the meaning of the sentence. There are potentially different kinds/types - so “kinds” is absolutely not bad English. Here is one source where kinds is used in plural but i found three other authoritative sources on a quick google. http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv310.shtml
You are right: I also wrote "kind", not 'kinds" and it was rejected. Sorry, but the program needs a refresher English 101 course
Sorry, Matt, but in this case it IS bad English, and nothing in your link contradicts it. Of course there are instances where it would be correct, but that's how language works. In fact, what you suggest, i.e. "What kind of things do you paint, old things?" is probably the best way to put it. Did duo reject it?
Sorry, but you need to be more specific then. I see nothing wrong with “What kinds of things do you paint, old ones?” And yes, Duo did reject it but it doesn’t now. This course was developed by volunteers so this kind of feedback is what makes it better over time. But a lot of Hungarians use it to learn English and the course is curated by volunteers who are generally not native English speakers, so we English speakers need to be careful about saying that something is bad English without explaining why. I think there are a range of possible precedent sentences where “kinds” would be perfectly acceptable. But we also need to be a bit flexible about the artificial context. For example, if the preceding phrase were “She paints cars?”, the translator doesn’t have the information to say “What kinds of cars does she paint? (e.g.) Old ones?” So in the absence of that information, the best translation in either language, without the precedent sentence, is “What kinds of things does she paint, one ones?”.
Do you mean “What kinds [of things] are you painting, old ones/things?” If so, that sounds perfectly ok to me.
You put your finger on it, Matt - this question lacks an antecedent. If it comes out of the blue, then "kinds" sounds wrong. But if we have a context, it can be perfectly O.K., of course. I do appreciate all the work that the volunteers have done for us, it is in fact quite a remarkable favour, so I suppose one had better just do what they want and be thankful.