You can find rules, but sometimes I don't find them convincing enough.
You just need to know that usually when the outcome is dubious, uncertain, unlikely, then you use the subjunctive.
However, some verb like "espérer" or "penser" is never followed by subjunctive even though it is questionable whether what is hoped or thought is certain or not. Some says, this is because the result is so desired that it is considered as certain by the speaker. Well, when you compare to Spanish that is more logical in the use of subjunctive, they use the subjunctive after the verb esperar, which is the same as the French espérer.
Unfortunately, only practice will tell you when to use subjunctive and when indicative future.