The difference between Indefinido and Imperfecto is probably your main struggle when using the past tense. Below are some examples of contrast situations:
Finished & Concrete Action vs. Durative Action
This is the most “grammatical” case.
Examples:
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Mi amigo vivía en Panamá vs. Mi amigo vivió en Panamá.
Actually, both cases provide the same information: I have a friend, this friend lived in Panama. The difference, as I told you before, is in how I perceive the action in the moment I imagine it.
A person who utters the first sentence (Imperfecto) is telling you some information that he or she has, but only about this moment as it occurred in the past. He/She doesn’t know or doesn’t want to say if this friend is still living in Panamá, is not still living in Panama, or what happened after that past moment.
On the other hand, the person who utters the second sentence (Indefinido) is providing a new information: that the friend is not living in Panamá anymore (vivió), and he/she knows that for sure.
Because of this, a speaker won’t be understood if he says *Visitaba España el mes pasado, because he/she knows for sure that the action is finished. Instead, this person must say Visité España el mes pasado.
Description vs. Narration 
If you are going to tell a story, remember this:
We use (Imperfecto) for descriptions, and (Indefinido) for discussing events. This is an easy one. If you want to describe how things were, how things used to be, in the moment you are speaking about, then you should use (Imperfecto).
Example:
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La casa era grande, tenía ventanas rojas, y delante de ella había un precioso parque. Las paredes estaban pintadas de blanco, y el tejado se podía ver una veleta con forma de gallo.
You are now very much involved in the situation. You can see the picture as if you were there. That’s the power of the (Imperfecto). It’s just like a photograph of how the past was in that moment.
However, you want your story to have some action as well. Here (Tense A/Indefinido) comes to your assistance. Anecdotes are mainly made using this form; fast and critical moments are constructed with it. It is with it that you really tell what happened. (Tense A/Indefinido) refers to actions seen as concluded, unitary, one-hit events.
Example:
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El niño golpeó la pelota, que cayó en el tejado del vecino. El vecino salió muy enfadado de su casa, y cerró la puerta tras él. Amenazó al niño con llamar a sus padres, pero el muchacho, muy valiente, le contestó que sólo era un niño. El vecino se tranquilizó, y, sonriendo, le devolvió la pelota al simpático joven.
This is storytelling. You can see the action going on, as if you were watching the movie of the past.
Repeated, Habitual Actions vs. Sequentially Successive Actions.
We use (Imperfecto) for the habitual actions, i.e. actions that were repetitive in the past. They can describe your daily routine, as in the following examples:
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Cuando era pequeño, me levantaba todos los días a las siete, desayunaba cereales e iba al colegio.
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Mis mejores amigos se llamaban Carlos y Juan. Mi padre nos llevaba los domingos a ver los partidos, y cuando ganaba nuestro equipo, lo celebrábamos cenando pizza en mi casa.
Notice they could be finished actions, but it doesn’t matter. What the speaker wants to remark on is the “repeatedness” of the events.
We use (Indefinido) for sequential actions. The same example that we use in the last section works here: a group of actions happening one after the other one, as in a movie, butonly one time each, for example:
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El niño golpeó la pelota, que cayó en el tejado del vecino. El vecino salió muy enfadado de su casa, y cerró la puerta tras él.
These are clearly one-time events.
Simultaneous Actions vs. Surprising Actions
Two actions that were happening at the same time in the past would be written in (Imperfecto), for example:
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Mientras mi madre cosía, la ropa se secaba al viento y los pajarillos cantaban en los árboles.
Just like in a picture, all actions are happening at the same time.
But sometimes, a current action (as in the past perspective we are speaking from) is interrupted by another one. We will use (Tense B/Imperfecto) for the first one, and (Tense A/Indefinido) for the interrupting one, for example:
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Mientras mi madre cosía, el gato saltó sobre las telas, y rompió el bonito jersey en el que trabajaba.
Specific situations: Unaccomplished wishes
When we had a plan that didn’t work, we use (Tense B/Imperfecto) in Spanish, specially with verb ir. For example:
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Ayer iba a estudiar, pero mi perro se comió los deberes.
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El jefe pensaba ascenderme, pero finalmente decidió ascender a su sobrino.
Specific situations: Politeness
This one is easy to remember. If you want to sound more polite, just use the (Tense B/Imperfecto) in your demands. It is much more well-mannered to say “Quería dos tomates y un melón”, for example, than “Quiero dos tomates y un melón”.
As simple as that. It just doesn’t depend on the vegetables you want to eat.