When do I use the subjunctive mood in French?

The subjunctive mood is used when expressing needs, desires, preferences, emotions, possibilities, opinions and suppositions. Putting it simply, it is used when something is subjective or uncertain.There are 2 triggers for the subjunctive:-the relative pronoun que-A change in the subject of the sentenceFor example, je veux que tu ailles à la bibliothèque.This phrase ticks all the subjunctive boxes: the subject changes from je to tu, que is present, and veux (coming from vouloir 'to want') indicates desire. Therefore, we use the subjunctive in the following phrase.