The imperfect = el imperfecto. The –aba and –ía tense. [a.k.a. pretérito imperfecto, copretérito]
Verbos –ar = --aba
Yo caminaba, tú caminabas, él/ella/usted caminaba, nosotros caminábamos, vosotros caminabais, ustedes/ellos caminaban. I walked (an unknown number of times) used to walk or I was walking, etc.
Verbos –er/ --ir = --ía Yo comía, tú comías, él/ella/usted comía, nosotros comíamos, vosotros comíais, ustedes/ellos/ellas comían. I ate (un unknown number of times), I used to eat or I was eating, etc.
This is a verbal tense that we use to describe the context, habitual situations, age, time, the weather, health conditions, most of the idiomatic expressions with tener (tener frío, tener calor, etc.), human relationships, actions that happened so many times that we have forgotten how many times they occurred... in one phrase: how things used to be in the past or at a certain period in the past. USUALLY, NOTHING (NO ACTION) HAPPENS IN THE STORY IF WE USE THE IMPERFECT, that’s why I emphasized describe at the beginning of this paragraph.
When I was 5 years-old, I lived in California.
Cuando tenía 5 años, yo vivía en California.
Yesterday it was raining cats and dogs when I got home.
Ayer llovía a cántaros cuando llegué a casa.
Open a novel in English on the first page and read the first lines. Usually, the first lines tell you about the context, the place, the time, the people, etc. This is IMPERFECTO.
Think of IMPERFECTO as something that happens, but we don’t know when it starts and when it ends, or how many times it happened.
But the difficult part between imperfecto and pretérito is when we use both at the same time.
[Link: youtube video, about the 6:20 mins.: http://www.youtube.com/watch?v=5ZRflz-93JA ]
Dave Chappelle has a very good routine about the difference between men and women. He says that a man tells a story kind of like this: “I went to the store, I bought a loaf of bread, came back home, made a sandwich and ate it.” But a woman tells the same story with a lot more of information: “I wanted to go to the store, but I had a headache, took an aspirin and then I went to the store. It was raining. It was cold. I got to the store. The guy was so handsome. Then I talked to him and he was mad about something... etc.” [These are not exactly Chappelle’s words, but they’re pretty close]
As you can see, according to Chappelle, men tell a story with action verbs only, while women add background and context. Men use the perfect preterite and women use perfect and imperfect. It is not 100% true, but it is a good way to explain the difference between preterite and imperfect.
Ask yourself how many times something happened or when exactly it happened. If you know it or can tell, then it is preterite. If not, it is imperfect. If something usually happens only once, it is preterite.
How many times did he die? Once! Then it is preterite.
Elvis used to live in Graceland.
How many times or for how long? According to the information given in the sentence, we don’t know. Then it is imperfect:
Elvis vivía en Graceland.
So, what is the difference between:
Elvis vivió en Graceland en 1970
Elvis vivía en Graceland en 1970?
The second one is either the context for a story, context information, or some trivial information. Maybe he lived there at some point in 1970.
The first one is a fact (preterite), maybe only sentence o comment. It's more emphatic. Maybe he lived there the entire year.
You can use either one without any problem. Unless you are telling a story in which the context dictates that this information is part of the description of how and where Elvis used to live, then you will have to choose. If you are making a list of facts that happened to Elvis, you can use the sentence in the preterite:
Elvis vivió en Tupelo en 1950, en Nashville en 1960, en la calle X de Memphis en 1965 y en Graceland en 1970.
(This is made up information, by the way)
The sentence in the imperfect can help you with a story like this,
Elvis vivía en Graceland en 1970 cuando compró dos aviones y tres televisiones.
(Again, made up information)
There are two more uses of imperfecto: parallel actions and paraphrases.
Parallel actions: The imperfect is the least important action and the important one is preterite.
Yo veía la televisión cuando mi hermana me llamó por teléfono.
I was watching TV when my sister called me on the phone.
Tenía dolor de cabeza por eso tomé Tylenol.
I had a headache that's why I took Tylenol.
Cuado estaba cocinando me corté el dedo.
When I was cooking I cut my finger.
It is possible to write the same sentences but with the preterite first:
Cuando mi hermana llamó por teléfono yo veía la televisión.
Tomé Tylenol porque me dolía la cabeza.
Me corté el dedo cuando estaba cocinando.
Paraphrases: when we re-tell a story, some verbs--that were in the present at the time the story happened—become imperfect.
Carolina said, “I want a burger.” (preterite + present)
If we re-tell the story, it goes like this:
Carolina said that she wanted a burger. (preterite + preterite)
Carolina dijo: --Quiero una hamburguesa. (preterite + present)
Carolina dijo que quería una hamburguesa. (preterite + imperfect)
I hope this helps.