Answer:
In order to complete this exercise, you have to fill-in the blanks in the sentences using the correct form of the preterite or imperfect past form of the verbs provided in brackets as appropriate for each case.
In general, the preterite will be used to describe completed actions in the past, while the imperfect is used to describe actions that occurred habitually in the past.
Note that in order to choose the correct inflection of each verb you have to pay special attention to the subject in each sentences.
Step-by-step explanation:
1. Marcos estuvo (estar) enfermo cuando regresó de sus vacaciones.
2. Dos enfermeras entraron (entrar) a la habitación esta mañana.
3. Todos los veranos nosotros íbamos (ir) a visitar a los abuelos.
4. Ayer, el doctor me recetó (recetar) unas pastillas.
5. De niño, ¿tú te caías (caerse) con frecuencia?