1. Cuando mi primo [empezó] (empezar) a trabajar, ya [tenía] (tener) 19 años.
2. Ella y mi hermano [se conocían] (conocerse) desde la escuela.
3. Mientras la cajera [revisaba] (revisar) la máquina, la ladrona [huyó] (huir).
4. El año pasado, las vacaciones [fueron] (ser) muy cortas.
5. [Había] (haber) mucha gente en la fila cuando yo [llegué] (llegar) para comprar los boletos para el teatro.

1. Pret: "Empezar" will be in the preterite unless you regularly began something in the past, for example, "I used to begin work at 8:00 am."; Imp: Age in the past uses the imperfect. Saying what age you were is a background description.
2. Imp: They knew each other. The key word here is "desde." Otherwise, "conocer" in the preterite means to meet. Example: Bob met Sally in 2005. / Bob conoció a Sally en 2005.
3. Imp: The cashier was doing an ongoing action which was interrupted by another action.; Pret: This action interrupts the ongoing action of the cashier.
4. Pret: This is simple past and is a summary of the vacation.
5. Imp: background description / ongoing setting; Pret: arrived once