"My grandfather is funny."
Translation:Mi abuelo es divertido.
If you're thinking of the "personal a," it's only used before direct objects. Here "my grandfather" is the subject.
I admire my grandfather. (Yo) Admiro a mi abuelo. (direct object)
You'll also see "a mi abuelo" used in sentences where it clarifies the indirect object: A mi abuelo le gusta correr. My grandfather likes to run. (Running is pleasing to my grandfather.)
The temporary vs permanent idea is often taught to frame your mindset. Don't get too caught up in it. Technically everything is temporary, so we can *never user ser!
Seriously though, think of it as feelings are expressed with estar and characteristics are expressed with ser. Being funny is one of his characteristics.