De quoi il parle est correct en français mais ne s'utilise pas seul. Cela doit être dans une phrase. Par exemple "je sais de quoi il parle", "je comprends de quoi il parle". Il y a peut-être d'autres tournures de phrases mais ce sont les plus fréquentes sont avec les verbes savoir et comprendre.
Quand vous voulez poser une question, la forme correcte est "de quoi parle-t-il ?"
"De quoi est-ce qu'il parle ?" est une forme qui peut être utilisée mais qui est lourde et qui n'est pas utilisée couramment par les gens.
I agree with you but it is awful to hear (and to read that as correct for Duolingo) for the french girl that I am. I try to explain again. If you want to ask a question (as the english translation gives us) you HAVE TO SAY "de quoi parle-t-il ?" (you want to know and you ask a question) When you say "de quoi il parle" you mean that you are speaking about somebody - who knows a subject and who speaks of this subject (il sait de quoi il parle) - who listen to someones who knows very welle a subject (je sais de quoi il parle, je comprends de quoi il parle). Is it clearer ?
Never use " de quoi il parle" to ask a question.
The correct form is "de quoi parle-t-il ?" - It seems difficult because we add the letter "t" but it is because it is not easy to separate "parle" and "il" when we speaks. So we add the letter "t" so it is easier to articulate.
"de quoi est-ce qu'il parle" is not useful for a french person. We do not really use the form "est-ce qu(e)" when we have to ask a question.
French learners have to be aware that in contemporary French, there are 3 ways of asking questions, depending on which register of speech is used:
formal: de quoi parle-t-il ? with the Verb-Subject pronoun inversion and the addition of a T for euphony
standard: de quoi est-ce qu'il parle ? this is commonly used in speech and in writing, even though the French Académie strongly recommends not using "est-ce que" after an interrogative word of 2+ syllables (there is some tolerance for "qui", "où" and "quand")
informal: de quoi il parle ? or il parle de quoi ?, that is with an indirect interrogative form, or a declarative construction. Both are improper French, but this is the way French people speak nowadays.
This post is not for the people learning on this site, but a small way for me to vent about something that really bothers me. When people tell me that I must be fluent in French because my first language is Spanish I feel extremely annoyed. I love French and everything French, but I find it very difficult to learn, specially to understand it. I have watched countless French movies, and I was lucky if I understood five whole sentences by the end of the film. In Spanish we pronounce every consonant and every vowel. It's easy to know if it is "he/she or they", for example, when we are working with verbs or singular and plural frases. All of the talking we do it's done in the same tone of voice. We don't have an "e" that have 3 or 4 different sounds. So, for all those people out there that don't think that, for us, it's a HUGE challenge to learn French ...think again.