Eres vs estas regarding fiancés
In the adjectives 1 lesson I had the phrase "Tu eres mi futuro marido."
But ser is for states of being that are more permanent, shouldn't the use of futuro imply that estar would be used - unless you wanted to drive home that you're not intending to marry? Maybe there's more nuance in the choice than I'm aware of...
This is kind of tricky. I would say because being someone's spouse is more of an identification than it is a condition, you need to use Ser. You could also use the future tense: "Tu serás mi marido" which means "You will be my husband."
When I was in high school, my teacher gave me these acronyms that helped a lot:
D - Description
O - Origin (where something/someone comes from)
T - Time
I - Identification
P - Position (where something is positioned, like "on top of the table")
L - Location
A - Action
C - Condition
E - Emotion
A less accurate, but general rule I follow for ser vs. estar is permanent vs. impermanent states.
Even though marriage in that sentence is dependent on conditions being met and is capable of being altered (i.e. divorce) it is an identity label rather than a conditional state like being tired or at home.
The permanency rule is an oversimplification since there are many smaller rules, so I would use those acronyms or memorize when you use one or the other verb. For instance, you would think that you would say "He is dead" using ser since death is pretty permanent but actually it is "El está muerto". (Any state that uses a verb participle uses estar like "to be alive/dead/broken/fixed/open/closed".)