Okay kiddo, so you know how sometimes we talk about things that might happen but haven't happened yet? Like saying "I hope it doesn't rain tomorrow." Or "If I were a superhero, I would fly." That's called using the subjunctive mood.
Subjunctive mood is basically a special way of talking about things that are hypothetical, imaginary, unlikely or uncertain. We use it when something is not a fact or a reality yet, but rather a wish, a suggestion or a possibility.
It is made by changing the verb in a special way in certain phrases, like "if", "whether", "that", "unless", and other conjunctions. For example, instead of saying "I am happy he is here", we would use the subjunctive by saying "I am happy that he be here." Or we would say, "If I were you, I would go outside" instead of "If I was you, I would go outside". See the difference?
It may sound a bit confusing at first, but it's easy to learn with practice. Just remember, when we use the subjunctive mood, we are talking about things that are not certain or factual yet, but rather just our hopes or imaginations.