11.5 dont , duquel / de laquelle / desquels / desquelles, de qui

11.5.1 When dont substitutes de of an opening phrase

Now we would like to discuss the situations when one relative pronouns can be substituted by another.
We have already mentioned that for fixed expressions with de this de changes to dont in the opening phrase of a relative clause..

Marie est une femme. J' ai parlé de Marie. Marie is a woman. I have talked about Marie.
  La femme dont j'ai parlé, c' est Marie. The woman about whom I have talke is Marie.
  Pierre est un menteur. Je me méfie de ce que Pierre dit. Pierre is a liar. I distrust the things Pierre says.
  Le menteur dont je me méfie, c' est Pierre. The liars whom I distrust is Pierre.

In this case dont cannot be substituted. Sometimes you might find the note that also de qui would be possible, will see that it is very, very seldomly used.

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