In this sentence, "des" is not an indefinite article. It's the mandatory contraction of "de + les". That's why we use "the" in English.
To make it less obscure, "parler de quelque chose" = "to talk about something".
It's the same for other verbs like "se moquer de quelque chose/quelqu'un" = "to make fun of something/someone".
So, if I wanted to say, "He speaks about red dresses," (without the "the") I couldn't say, "Il parle des robes rouges?" This is certainly how I would have said that, which would force it to be a correct English translation in this exercise. Granted, I haven't lived in France in 9 years so I'm a little rusty, but to me, "Il parle de robes rouges" just sounds wrong.
"Il parle des robes rouges" means that he talks about specific red dresses we talked about earlier in the discussion. So in English it would be "He talks about the red dresses".
"Il parle de robes rouges" is perfectly correct French, and means that he talks about red dresses in general. In English : "He talks about red dresses".
As I said in one of my posts above :
"to talk about something" = "parler de quelque chose."
This basic form can change if articles are used after "de", like in this exercise : "des" is in fact "de + les" and not the indefinite article "des".
The resulting forms are :
des, de la, du, de l', d'un, d'une, and de (which is unchanged when combined with the indefinite article "des").
Also, keep in mind that the fact that "Il parle de robes rouges" is perfectly correct French doesn't mean that it's a correct answer for this exercise.
"de" is used for MANY things, and I can't cover all of them here. Usually, it doesn't have any gender or number, except when it's used as a noun (and that's very rare).
You can learn more about "de" here :
However, they don't cover everything, so I strongly suggest that you do some research if you want to know more.
Arthur - Many thanks for your quick reply - it is appreciated: All I have to do is digest your info: "de certaines robes rouges" is fairly self explanatory, though "Il parle de quelques unes des robes rouges" is rather less intuitive. It seems a certain inherent knowledge of the mechanics of French is almost a prerequisite!!
Well both sentences work on the same principle, they consider "some" designating only a part of all the red dresses, using [which his wife used to wear] as a criteria to filter which ones we'll talk about.
In "Il parle de certaines robes rouges.", "certaines" is an adjective to "robes", which by itself already means that we won't talk about all red dresses.
In "Il parle de quelques unes des robes rouges.", "quelques unes" is a pronoun, which tells us that we'll use a part of "something", but if we wouldn't link it with "robes rouges", we wouldn't know what would be that "something". That's why we use "de + les = des" to specify that "quelques unes" actually is a pronoun designating "some of the red dresses" ([which his wife used to wear]).
If it can help you with the second sentence, if we had another structure, for example this one: "Il a des robes rouges, j'en prends quelques unes." then you can see that we don't need to specify what "quelques unes" refers to, since "robes rouges" was introduced already as the main object of the sentence.
As long as [which his wife used to wear] is known to the speaker, it would be "Il parle de certaines robes rouges." or "Il parle de quelques unes des robes rouges.". Otherwise, we would usually specify in the same sentence the criteria used to filter the objects (in this case [which his wife used to wear]).
Really, indefinite article though??? "Il parle des robes rouges" How is that not, 'He speaks of red dresses?' But is considered correct as, "He speaks of THE red dresses?!!!!" Tell me why the indefinite article is essential in this sentence, the meaning is not lost due to the omission of "the". I am completely annoyed.
"Parler de" = "to talk about". As an English speaker, this makes sense to me. I prefer "He talks about the red dresses". Why not omit the "the" in English? Because it represents [parler] De + les (definite article combined with de to make des). So I imagine the sentence is about red dresses in general or particular dresses known to the speaker.