In relative clauses expressing a wish or an irreality the subjonctif is to be used.
Examples
J' aimerais avoir une voiture qu' on puisse plier.
I would like to have a car that one can fold together.
Imaginez-vous un état, dont le gouvernement
soit
compétent.
Imagine a state, where the government is competent.
J' aimerais avoir une voiture, qui n' ait pas besoin d' essence.
I would like to have a car that does not need fuel.
If the main phrase contains a superlative (something biggest, greatest, best etc.) usually the subjonctif
is used, because it is not an objektive statement but a subjektive
evaluation.
Examples
C' est la femme la plus belle qu' on puisse s' imaginer.
This is the most beautiful woman, that one could imagine.
Il est le seul dans la classe qui ne sache pas encore écrire.
He is the only one in his classe, that cannot yet write.
