Interrogative pronouns are the pronouns used to form an open question (Who, What, Which). Further, we have interrogative adverbs that are used for this purpose, but we will come back to these in a bit later.

In previous chapters we have already discussed the fact that there are two types of pronouns, the adjectival (used like an adjective before the noun) and the substantival (used instead of a noun). Here an example with demonstrative pronouns.

adjectival: These flowers are beautiful.
  substantival: These are beautiful.

In the group of interrogative pronouns there are some used in both forms, in adjectival and in substantival.

adjectival: Which dress do you like more?
  substantival: Which one do you like more?

First we have the ones that ask for who (qui) or what (que, quoi). They cannot be used adjectival, they always represent a noun.
Then we have the pronouns which (quel, quelle, quels, quelles) in relative pronouns (as we have the ones mentioned above). The specialty of these are that they ask for a member of group, which needs to be known by all parties of the conversation.

Following now an overview of the interrogative pronouns, also with the alternatives of the construction with est-ce que or a possible inversion (possible in accusative/direct object).

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