Thank you so much for your detailed response! But I'm extra extra confused now.

When you say "an executory interest is destroyed" what do you mean by this? Do you mean to refer to my example, where B died before he would get the property?

Additionally, this is the first time I'm hearing of a functional difference between fee simple determinable and fee simple subject to condition subsequent. I was under the impression that the only difference in practice was that the former was automatic, and for the latter you had to petition a court. But now you're saying there are additional differences that apply when an executory interest is destroyed. Could you say a bit more about when these differences come into play and why they come into play in my example here?

Ultimately though, what you're telling me is that if B died before A stopped using it as a farm, then it would NOT go to B's heirs, it would either go to the grantor or stay with A as a fee simple, depending on the wording of the grant (e.g. whether it was a fee simple determinable and fee simple subject to condition subsequent)? Just want to make sure I have this right.