The easiest way to think of it is this: Was it a 1 time, finished action? Such as, Naci el 8 de febrero de 1986.
Then you use the preterite. If you can put an exact day and time on the action - it needs to go in the preterite.
The imperfect us used for habitual past actions.
Things you used to do.
Or were doing when another action interrupted it.
And descriptions that have no active person.
If you can say a frequency adverb, like always, sometimes, never, similar to the present tense, then use the imperfect.
Also, descriptions, weather, dates, age, etc. always go in the imperfect.