In general, the difference between the two simple past tenses of Spanish is that the preterite tense refers to an action that happened at a specific time while the imperfect tense refers to action that has no definite ending time. (Of course, there are exceptions.)
Sometimes, as explained in our newest lesson, on meanings of verbs in the past tenses, the result of choosing one tense over another can be a difference in meaning (at least to those who speak English) of the verb. For example, using conocer in the imperfect tense can be used when talking about knowing someone in the past (an act that doesn't occur at a specific time), but the same verb in the preterite typically is used to refer to meeting someone (an act that does).
Comments
RE: http://spanish.about.com/od/verbtenses/a/past-meanings.htm
You wrote:
Thus “Mi vida fue feliz” (preterite) and “Mi vida fue feliz” (imperfect)
Could you elaborate? I believe you meant two different forms of the same verb (or the same form of two different verbs)..
Then perhaps, I am wrong.
Thanks.
— A Student