1. Ser vs.\ Estar
Ser and estar both mean ``to be'' but split by permanence/identity (ser) vs.\ state, condition, or location (estar); the CLEP tests this contrast relentlessly across every section.
Ser: identity, origin/nationality, profession, time/date, material, possession, inherent characteristics, and the location of an EVENT (``El examen es en el aula 3''). Estar: location of people/objects, ongoing states/conditions, health, and the progressive (estar + gerundio). Meaning-shift adjectives: es aburrido (boring) vs.\ está aburrido (bored); es listo (clever) vs.\ está listo (ready); es malo (bad) vs.\ está malo (sick/spoiled).
The location of an event uses ser (``La fiesta es en mi casa''), but the location of a person or object uses estar (``Ella está en mi casa'').
Discrete-sentence items swap ser/estar with a meaning-shift adjective, producing a grammatically valid but semantically wrong sentence --- picking the wrong pairing is the trap, not a conjugation error.
ser = identity/origin/time/event-location; estar = person/object-location/condition/progressive.