The "completed vs incomplete action" explanation that you read in grammar books is a little too simplistic. For example, you say "Cuando era niño..." (When I/he was a child). Obviously, I'm not a child any more, so technically this is a completed action, but we still use "era" for this. Also: "Mi madre era una mujer muy bella." She's not beautiful any more (and the action is completed if she suddenly got ugly, and incomplete if she died while still beautiful), but most speakers will use "era" here regardless of how it came about that she is not currently a beautiful woman. Why? It is because the action "continued" for a long time and is not considered an event. Imperfect is often used for "continuing action in the past", which basically means, actions that went on for a relatively long time, even if they did in fact finish. Other types of actions in the past, however, usually take preterite, even if they lasted for years. ( "La Segunda Guerra Mundial fue un conflicto militar global que se desarrolló entre 1939 y 1945." The difference here is that this war started on a specific date, ended on a specific date, and is considered a single event. Another "guideline" that often works is: if you can use the English words "used to", then you can use the imperfect. I say guideline because, although in certain cases virtually 100% of people will agree that it is one or the other, in many cases that consensus is not universal, although there will usually be a fairly strong majority of users preferring one form over the other in any particular case. If you do a web search for a simple phrase (in quotes) using one form, and then the other, you will quickly figure out which is the preferred form for that particular case. After doing several of these searches, you will become increasingly familiar with the patterns associated with each usage. There is, unfortunately, no easy explanation that will cover every single case.

Hola sweetsatsuki, How are you? It's a little confusing the using of them, but you have present this "fue" is a verb in the Simple Past Tense and therefore indicates that a action was ended in its totality "era" is a verb a in the Imperfect preterite and indicates that a action wasn't ended in its totality. For example: Yo fui un estudiante de medicina= Acción terminada, ya es un estudiante graduado y titulado en su oficio. Yo era un militar = Acción no terminada, algo ocurrió que no permitió que siguiera en su oficio.

Preterite in Spanish is for completed, specific actions, and the imperfect is for describing the past (how things used to be).... examples: Él era mi unico amigo cuando joven. Ayer fue el 13 de abril, hoy es el 14. Ella fue la que rompió el vaso. Yo era parte de un grupo de musicos, pero ya no. I hope this helps 8]