Anyone who knows the difference between indicative and subjunctive?
The indicative is strictly used in any sentence expressing the reality (and in questions though). The subjunctive is used whenever something is stated which does not belong to reality, like a hypothesis. Hence negation needs the subjunctive, as when you state that "The dog is not happy." the dog's being happy is excluded from reality. The subjunctive often carries the meaning of "could, would, may, might" in English.
To add to what Xenon- said, it also depends on the language how it's used. Not every language uses the subjunctive in the same way. My native language has a subjunctive mood, but it was still the most difficult verb conjugation for me to pick up in other languages because of those differences.
In the High Valyrian course, so far it's been used for negation, and also in subordinate sentences where the verb in the main clause is "jaelan". The latter is pretty typical for the subjunctive - stuff like "I wish/hope" + a subordinate sentence in the subjunctive mood.