Let's stop by the kitchen of the Di Carlo mansion, setting of preparations for the big gala in Muñeca Brava. The maids are very excited. They want to get a detailed description of how Mili looked as she made her Cinderella-like debut. Notice that Socorrito uses the imperfect tense of both ver (to look) and bajar (to go down, to lower, to descend) when she asks:

...Contame, contame,

...Tell me, tell me,

¿cómo se la veía cuando bajaba de la escalera?

how did she look as she was walking down the stairs?

Caption 1, Muñeca Brava - 41 La Fiesta

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If you've ever heard anything at all about the imperfect tense, it's that it applies to past actions that are not completed or that are ongoing. We see that quite clearly above in the case of bajaba; Mili "was walking down," an action that was ongoing at the time. However, another rule of the imperfect, one less bandied about, also comes into play here: the imperfect is employed when describing two or more simultaneous past actions. Socorrito wants to know how Mili "looked" (using the imperfect veía) as (at the same point in time) she was going down the stairs. 
 

With her usual enthusiasm, Mariposa definitely puts them in the moment when she answers:

Socorrito, ¡no sabe lo que era! Parecía una princesa.

Socorrito, you can't imagine! She looked like a princess.

Caption 2, Muñeca Brava - 41 La Fiesta

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There is yet another well-documented use of the imperfect that we can cite here: its use to "set the scene" or provide background information, especially at the beginning of a larger story. She uses the imperfect era (from ser, to be) when she says ¡no sabe lo que era! which literally translates to "you don't know how it was!" And she employs parecía (she looked like), which is an imperfect conjugation of parecer (to appear as/to look like/to seem like). Mariposa is setting the stage for the fairy tale taking place in the ballroom, and doing so in much the same way one would recite an actual fairy tale (which is no surprise if you remember that Muñeca Brava is a retelling of the Cinderella story).

The start of your average ghost tale or mystery story makes a good illustration of using the imperfect to paint a background picture:

Era una noche oscura y tormentosa, llovía y unos pájaros cantaban a lo lejos.
It was a dark and stormy night. It was raining and a few birds were singing from a distance.

[Note that in Spanish one can also use the past continuous tense, for example estaba lloviendo (it was raining) or estaban cantando (they were singing)—but it would not likely be used by native speakers when setting a scene or providing a backdrop. We'll look at the past continuous, aka past progressive, in a different lesson.]

More well-known to the average student of Spanish is the use of the imperfect to refer to a habitual or repeated action in the past. We saw an example of this in an earlier episode of Muñeca Brava when Milena says to Louise:

Sí, antes nos veíamos siempre.

Yes, we always used to see each other.

Caption 68, Muñeca Brava - 18 - La Apuesta

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The other simple past tense in Spanish (called "simple" because its conjugations are only one word long) is known as preterite and is used for past actions that are completed and non-habitual. We find an example in a recent music video from The Krayolas:

Cuando yo la vi por primera vez me enamoré en un dos por tres

When I saw her for the first time I fell in love with her instantly

Captions 1-2, The Krayolas - Little Fox

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The singer uses the preterite vi (saw) instead of the imperfect veía (was seeing/used to see) because he is talking about a specific, completed instance of laying eyes on someone.

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Read more interesting things about the imperfect on the 123TeachMe site and be sure to visit Spaleon to master the imperfect conjugation of all verbs.