Aurélie, both que and qui can be used for which and that. And we qui when the following word is a verb or reflexive pronoun.

Question

Il veut visiter le château qui est au milieu de la forêt. 'visiter is the verb but it does not follow it is before qui so why is que in this question wrong?

Answer

Well, the verb "est" follows right after "qui". It is the relevant verb for the relative clause.

Answer

Hi Fiona,
You could have a subject pronoun or a name  after 'que' as in the following examples :
C'est le château que Claudine a visité = That's the chateau which Claudine visited
Il veut revisiter le château que j'ai déjà vu plusieurs fois = He wants to revisit the chateau that I have seen several times already
Hope this helps

Answer

Thank you both Chris and Cecile (sorry Cecile my accent is not working). I am still wrapping my head around these, I will practise more.