imperfect - past action important or continuing in the present

and Marco wants to cut it off and forget it.

   preterit - distinct action which is over and done.

Marco said “Fue un día terrible.”  If he continued the sentence so that the horrible day  was background for the events of the day would he have said “era un día”. 

For example

“(Era/fue)  un día terrible, perdí mi trabajo, me esposa me dejó y mi perro se morrión”

My guess is that in this case “era” is correct but I am amazed that after all this time the preterit and imperfect still can confuse me.

http://teachers.net/mentors/spanish/topic22626/8.13.09.13.13.19.html

Thank you for the link

 yo estaba vestido de superman (2.40 ) is correctly referred to as the imperfect (i was dressed as) although, as you noticed,  at (3.08 ) the same phrase is referred to as the preterit (i dressed as)

 strangely what he fails to mention in this lesson is that what is usually known as "el preterito" is infact el pretérito indefinido or el pretérito perfecto simple as opposed to  el pertérit imperfecto. i.e. they are both preterit tenses.

Still, well spotted, I have no doubt that JP meant to say imperfect at 3.08 

Also, it can be helpful to think of the imperfect as "an ongoing movie in the past" and the preterite as a "snapshot or single foto in the past."

I find that for me personally, the biggest problem is thinking about it too much!  If I trust my gut, relying on what I know feels right, I rarely make mistakes with either. 

perhaps its an insignificant detail but i was long misled by explanations that focus on the action itself . It seems to me that the most important factor in choosing between the simple past and imperfect is personal choice. it simply depends how one wishes to present an action to ones interlocutor. 

Whether the action was short, long, repetitive habitual etc does not determine the choice of tense. You must decide which aspect of the action you wish to communicate.  turning on  a light is about as instantanious an action as one can imagine but i can still choose to say "i was turning on the light when..... or i would turn on the lights every day...i used to turn on the lights...

nor is it determined by how one percieves an event, the reason we dont need a simple present tense is that we always experience the length of new actions. leo could not experience his birthday party in the preterit. however from the vantage point of the present one can choose to communicate past events with a key descriptive frame or to replay the scene 

You folks are crazy :-D I laughed a lot on these "face into the cake" examples. Great video!