Here's how you introduce a relative clause including a prepositional verb or verbal expression with of/about in English:
The person about whom I'm talking is coming later.
The person (whom) I'm talking about is coming later.
In French, not only is the structure different, but you also have different relative pronouns (dont, de qui, duquel) available to introduce relative clauses with prepositional verbs or verbal expressions with de*.
Let's learn when to use which.
How to use "de qui / dont / duquel" in French
First of all, note that
The boy whom I'm speaking of
-> You cannot say any of these:
Le garçon qui je parle de
Le garçon je parle duquel
Le garçon je parle dont
La fille laquelle je parle de
Now let's have a look at our different pronouns.
Dont = of/about which and of/about whom
The boy of whom you are speaking is very nice.
De qui = of/about whom
Duquel, de laquelle, desquel(le)s = of/about whom and of/about which
However, there's one case where you can favour duquel, etc and that is when using dont is ambiguous, and you want to be specific as to who/what is being referred to.
(Note that you can still use dont or de qui (people only) in such cases.)
* Here's a non-exhaustive list of French prepositional verbs and verbal expressions with de:
See also Dont = Whose (French Relative Pronouns), Dont = Including/[quantity] of which (French Relative Pronouns) and Duquel/de laquelle/desquels/desquelles with prepositional phrases with "de" (French Relative Pronouns)
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Examples and resources
(Sacha Guitry)I guess a woman's past from the way she holds her cigarettes, and a man's future from the way he holds his drink/liquor.
(Sacha Guitry)
lit. The shoes of which you're in need are in the cupboard.
The boy of whom you are speaking is very nice.
The witch of whom he is scared lives here.
The witch of whom he is scared lives here.
The boy of whom you are speaking is very nice.