Ser, Estar, el Pretérito, y el Imperfecto.
Hoy en mi examen de español había una pregunta que dice : Mientras veíamos la película, alguen llegó a la puerta (llegó was a fill in the blank which i completed correctly) entonces se escirbe: _______ un estudiante nuevo. I filled in fue, but the answer key said era. I kept using el pretérito because of llegó in the sentence before.
Could someone explain to me why using the imperfect is correct in this context? ¡Muchas Gracias!
3 Answers
Llegó was used because preterite is used for a past action that interrupts one that had been ongoing. The person entering interrupted the action of watching a movie.
And Noetol is right--the person who entered was a student, ongoing. It was part of their nature, their being, so it would be imperfect.
Just to conjecture here, llegó was proper because it referred to a specifically defined action in the past. Era referrs to the student himself - he was a student before and after he came to the door, a continuous description with no defined beginning or end.
Only my take on it.
As Noetol pointed out the preterite is good for completed past actions, the person being the new student however was not an action but a description so ser/imp is the one to use 
