1) Ce, tout
and rien are in gender and number undefined. Lequel
/ laquelle / lesquels / lesquelles can only refer to things that are defined in gender and number. This means that lequel / laquelle /
lesquels / lesquelles cannot be used in these cases. Here, we use quoi as relative pronoun. There is a chapter regarding these special cases here.
Example
C' était tout sur quoi il a fondé son jugement.
This is everything on what he has based his judgement.
2) duquel / de laquelle /
desquels / desquelles anstatt dont
Usually dont is used for references with the preposition of.
However, if the referred object is on a place not directly after the referred object,
auf duquel / de laquelle / desquels
/ desquelles is used.
Example
L' homme dont la femme est partie en Italie lit un livre.
The man whose wife has gone to Italy reads a book.
but
Examples
L' homme avec la
femme duquel
il avait parlé, a disparu.
The man with whose wife he has talked, has disappeared.
La femme avec le
mari de laquelle
il avait parlé, a disparu.
The woman with whose husband he has talked, has disappeared.
3) We have said that
de changes to dont when it appears in a phrase of a relative clause. This is not the case when a reference is made to something abstract, where we use "of what" in English.
