"Are you looking for her?"
Translation:Ti őt keresitek?
So, for the third person, you need to use the definite conjugation. "Őt keresed?".
Öt keresed? Think of it as an object when the verb gets a "d" or "t" suffix.
Mit keresel? Here the question word "mi" = what, has a 't" at the end, therefor you don't need to modify the verb.
if you are talking about a definite object/person you need to use the "d" suffix: "A kutyát keresed?" - "You are looking for the dog?" (there is a specific dog (yours, or implied by the context) you are looking for) "Egy kutyát keresel?" - "You are looking for a dog?" (in general, it doesn't matter which one, just be a dog) In this case "ő" - "she" is define, narrowed down to only one specific person, hence the "d" suffix.
In the question "Mit keresel?" the object is not precisely define, therefore the generic "l" suffix.