I looked it up and was sort of understanding why you might use “sea” instead of “es,” such as in the case where you might be describing something, but the description you are using isn’t hard fact, it’s more opinion. Subjunctive. So it’s the difference between saying, “Mi papa es un hombre alto,” which is a fact, that he is tall, and “Mi papa sea genial pero no sea perfecto.” That’s your opinion of your dad.
However I have also been taught in school that it is okay to say “Mi papa es genial pero no es perfecto,” to describe traits of someone.
I just watched a video where someone said, “Me alegría que mañana sea viernes.” But tomorrow would literally be Friday, there’s no opinion about what day of the week it is. So why wouldn’t it be “es”?
I might just have it all wrong! But thank you for any insights! 🙂