question said I was wrong when I wasn't

Question

Esta ________ mi antigua escuela.This used to be my old school.(HINT: Conjugate irregular “ser” in the imperfect past/El Pretérito Imperfecto. Example: "ella tenía" = she had/used to have)Esta solía ser mi antigua escuela.  I'm pretty sure that I'm right and it puts "era". I know that "solía ser" is even more accurate than "era".

Accepted Answer

Hola Alexander
Your answer would be right if that hint wasn't there indicating what this question is testing, which is in this case the conjugation of ser in El Imperfecto (not soler in El Imperfecto). 
I hope this clarified it.
Saludos
Inma

Answer

The question gave the hint to use ser in the imperfect not the construction soler + infinitive. The verb soler means usually do”, or “used to do” so it wouldn't fit in this sentence anyway.