When do I use the subjunctive mood in French?

The subjunctive is associated much more heavily with the subject of the sentence than any other mood. This means the the personal feelings of the speaker play a huge role in the meaning of the verbs: hence, subjectivity. Putting this into practice, it is important to distinguish between the uses of the indicative mood versus the subjunctive mood: use the indicative when what you are trying to convey is generally seen as a fact or a common truth, there is no personal feeling attached to what you are saying; use the subjunctive when you are unsure about what is being said, or if you feel a certain way about what you are saying, and wish to convey this in your language. For example, 'Le ciel est bleu.' is a common truth, a fact: everyone knows that the sky is blue, whereas in the sentence 'Je deteste que mes professeurs soient si vieux.', the fact that you hate something is a fact, but the personal feeling is attached to your teachers' being old: you hate it. Another use of the subjunctive is to denote an air of uncertainty to a statement. For example, 'J'espère qu'il soit mieux demain'. You hope that it gets better tomorrow, but you're not sure if it will or not, so you must use the subjunctive. Finally, use the subjuctive when giving orders in indirect sentences, e.g. 'Je veux que tu fasses tes devoirs'.