1. EL IMPERFECTO VS. EL PRETERITO
When to use them
and what they can mean

2. “used to” = IMPERFECT
Conjugate any action in the imperfect to mean that someone used to do something on a regular basis.
You will often precede this use of the imperfect with an expression like de pequño or de niño.

3. “was/were –ing” = IMPERFECT
You will usually use this meaning when you want to set up a scene and when some other action is going to interrupt the “-ing” action.
For example: I was reading peacefully when an anvil fell on my head.

4. time and weather = IMP
No debates here: if you’re talking about what time it was (a specific time, late, morning, early, evening, whatever!) it will automatically be the imperfect.
Anything weather related – unless you give a specific time period – is the imperfect.
example  Son las diez y veinte. / Eran las diez y veinte.
example  Nieva. (now) / Nevaba (past).

5. physical, mental, emotional = IMPERFECT
PHYSICAL talking about how someone was feeling – estar (sick, well, tired, etc.)
MENTAL  wanting something, knowing something, thinking something
EMOTIONAL  being sad, happy, confused, scared

6. describing a person, place, or situation in past = IMPERFECT
When you are describing someone or something using an adjective in the past, you will usually use the imperfect.
e.g. My grandmother’s house was big and pink.  La casa de mi abuela era grande y rosada.
e.g. My teacher was nice.
Mi profesora era simpática.

7. imperfect verbs These verbs are almost always imperfect: estar  to be
parecer  to seem
pensar / creer  to think
querer  to want
sentirse  to feel (REFLEXIVE REFLEXIVE)
tener  used with certain expressions (calor, frío, hambre, sed, sueño, etc P. 237)

8. PRETERITE = completed action in the past
An action that happened and is over with.
e.g. I ate breakfast. (Breakfast is over.)

9. PRETERITE = interrupted action
Somebody was doing something WHEN something happened.
e.g. We were taking notes when a space ship landed outside.
interrupted by
preterite
imperfect

10. preterite verbs These verbs are almost always preterite:
empezar / comenzar  to start
terminar  to end
llegar  to arrive
entrar  to enter
salir  to leave / to exit
despertarse  to wake up
acostarse  to go to bed

11. HABÍA vs. HUBO había is imperfect version of “hay”
Use había when you want to describe what there was or what there were in a particular situation.
No había tarea ayer.
Había muchas personas en la fiesta.
hubo is the preterite version of “hay”
Use hubo to say that something happened or occurred. You will put it before the event that happened or occurred.
Hubo una explosión en el edificio.
Hubo un terremoto terrible en California.

12. Sum up…
In general, think about the imperfect and the preterite like this:
Preterite: Used for completed actions in the past. The actions was done and over with.
Imperfect: Used for ongoing or continuous actions or states in the past. These often have no specific beginning or end.