Does croire require the subjunctive?
Croire may require the subjunctive, depending on whether it is used affirmatively, negatively, or interrogatively:
Je crois qu’il sait la vérité.
I believe he knows the truth.
Je ne crois pas qu’il sache la vérité.
I don’t believe he knows the truth.
Crois-tu qu’il sache la vérité ?
Do you believe he knows the truth?
À noter : The above is correct according to Le Bon Usage and Le Grand Robert, among others. However, so many native speakers mistakenly use the indicative when croire is in the negative or interrogative that many now believe the subjunctive to be optional. Others disagree.