When do you know when to use preterit/imperfect?
I somewhat understand how preterit is for a specific moment and imperfect is for something that is on going, but I don't exactly understand all the rules.
Here are some ways I think about preterit and imperfect.
Preterit usually indicates an action that had a definite starting and end time. Sometimes this is stated in the sentence. In, ``Yo caminó por 20 minutos anoche´´ we are told a time frame, twenty minutes. We now know the action started and stopped, and furthermore, it happened last night (anoche). Unfortunately, sometimes your understanding of this will have to be contextual. If someone asks where you went last night, and you said you walked to the park, you would answer similarly above. You walked to the park, you walked back, end of story. Here´s where imperfect tense comes in.
Let´s say someone asks you about a time in your past, but it´s not yesterday. It´s the time you lived in a different town, and they want to know what you used to do. Unfortunately, if you´re anything like me, your life might be boring and you might say you used to walk to the park. Instead of time being a capsule here that denotes a beginning and an end, we have multiple events linked together. Plunging into your memory, maybe you walked to that park a hundred times! Who knows when you first went? But back to the question. Someone asks you:
¿Qué hacía como niño?´´ (What did you used to do as a child?´´) Boring us responds:
Yo caminaba al parque.´´ (I used to walk to the park.´´)
There is a caveat here: Imperfect tense can often be interrupted by a preterit tense. Let´s complicate the memory. Remember that park you used to walk to? One day you walked there to discover all the beautiful trees and swingsets had been bulldozed. By that time next year it was a set of high-rise condos. In this case, an imperfect verb
used to walk´´ was interrupted by a preterit verbbuilt some condos.´´ So this woman keeps interviewing you about your childhood, wanting to get to the bottom of this fascinating story, and you throw your hands up and say, ``¡Yo caminaba al parque hasta se construyeron los condominios!´´
Hence, it´s not accurate to say all imperfect verbs are ``ongoing,´´ in fact, like most good things, they end too. But in general, we´re asking a question of vagueness. Most things you have done more than once, especially repetitive tasks or activities, end up in the memory blur of imperfect. The biggest exception is when they end sharply-- but only because they were interrupted by a definitive action, a preterit.
Hope that helps!