"My grandfather is at the bank with my father."
Translation:Mi abuelo está en el banco con mi padre.
at vs in seems to be troublesome! "at the bank" can mean "at the door of the bank" or "inside the bank" but "in the bank" means inside the building not at the door! I hadn't realized how imprecise at/in can be. Study a foreign language and learn about your own language!!
Why isn't "Mi abuelo está al banco con mi papá" accepted? At the bank vs. in the bank. They mean slightly different things, and "at the" is "al".