Spanish inserts a tiny word -- a -- right before a direct object when that object is a specific person or animal, and drops it for everything else. English has no equivalent, so it's easy to forget, but leaving it out in front of a person is one of the fastest ways to sound like you're still translating word for word.
Use it whenever the direct object is a specific person or pet. Skip it for things, places, and people who aren't specified.
Direct object is a specific person (or pet)
Direct object is a thing, place, or unspecified person
Busco al médico que me operó, no un médico cualquiera.
I'm looking for the doctor who operated on me, not just any doctor.
Same verb, same kind of object -- but al (a + el) marks a specific known person, while un médico leaves it open to anyone. The personal a tracks specificity, not just whether the object is human.
Take the quiz and see if you can spot where the personal a belongs.