Questions about the subjunctive
Q: Why is it il faut que tu fasses and NOT *il faut que tu fais?
The expression il faut que is always followed by the subjunctive (and it is a VERY common expression).
Q: Why is it il faut (or je dois) and NOT je faux?
The verb falloir is never used with je as the subject. The only subject it can have is il. When describing something you need to do, use the expression il faut que (e.g.: Il faut que je parte demain) or je dois (e.g.: Je dois partir demain). The verb falloir is only conjugated with the pronoun il, which means "it". In other words, when you do write Il faut, it doesn't mean "he must". To say "he must", you should use the verb devoir, e.g.: Il doit partir demain ("he must leave tomorrow). Note that the most common English translations for Il faut are "We need to ..." or "It is important to".
Q: Why is it j'espère qu'il viendra and NOT j'espère qu'il vienne?
We admit that this isn't particularly logical, given that espérer expresses a desire and verbs that express desire usually trigger the subjunctive. However, that's not how standard written French works! When using espérer, use the future of the following verb.
Q: Why is it ... après qu'on le fait and NOT ... après qu'on le fasse?
The expression après que is followed by the indicative, while avant que is followed by the subjunctive. This makes sense in a way because we can be more certain about events after they happen than before.
Q: Why is it je crois qu'il peut venir and NOT je crois qu'il puisse venir?
The verb croire is followed by the indicative in affirmative (positive) sentences, but followed by the subjunctive in negative ones (the idea being that there is less certainly in the latter case).
Q: Why is it je ne pense pas qu'il le sache and NOT je ne pense pas qu'il le sait?
The verb penser is followed by the indicative in affirmative (positive) sentences, but followed by the subjunctive in negative ones (the idea being that there is less certainly in the latter case).
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