(New notes at top; old below)

SITUATIONS WHEN TO USE THE SUBJUNCTIVE FORM OF VERB

1- VOLITION (IMPOSING ONE'S WILL ON SOMEONE)
           Mi madre quiere que yo estudie más.

2- DENIAL
           No es posible que ella tenga 30 años.

3- HOPE OR POSIBILITY
           ¿Esperas que yo venga tarde?
           Es probable que nieve mañana.

4- DOUBT
           Ellos dudan que hables alemán.

5- UNCERTAINTY
           No es cierto que Pablo compre un coche.

6- EMOTION
           Siento que María esté enferma.

Preterite tense is the simple (one-word in English) past.  The preterite refers to an action completed in the past:
 

I ate an apple. Comí una manzana. (The apple is gone.)

He read the book. Él leyó el libro. (He finished reading the book.)

Imperfect tense is a compound (more than one word in English) past.  The imperfect refers to the middle (after the beginning, before the end) but while the action was on-going.  NOTE!  The action most likely was completed at some point, but the speaker’s interest is in the middle.  Most often this type of action is expressed “was/were talking (eating, driving), but ¡OJO!, some people use the simple past here when they actually are referring to the middle.
 

I was sleeping at 8 AM.  Yo dormía a las 8.  (I went to sleep before       8, and woke up after 8.)

They were studying.... Ellos estudiaban.... (I don’t know when they  started or finished.)

The imperfect tense is also used for repeated actions (used to have, would drive) in the past.  ¡OJO!, some people use the simple past here when they actually are referring to a repeated action.

I used to visit (would visit) my grandmother almost every Sunday. Yo visitaba a mi abuela casi todos los domingos. (Even without “almost every Sunday”, visitaba tells you it was a       repeated action.)

Preterite and Imperfect in the same sentence.
Often the two tenses can be combined with each other.

While Sam was in the room, I ate an apple. Mientras Sam estaba en el cuarto, comí una manzana. (The apple got eaten while Sam was there, but I don’t care when       he arrived or left.)

I was sleeping at 8 AM when Beatriz called (phoned) me. Yo dormía a las 8 de la mañana, cuando Beatriz me llamó. (My sleeping, which was in progress before 8, was interrupted       by the phone call.)

Carlos read War and Peace while he was in the 9th grade. Carlos leyó La guerra y la paz mientras estaba en el noveno       grado. (He was in the 9th grade both before and after he started and      finished the book.)

Multiple preterite verbs or multiple imperfect verbs in the same sentence.
A series of either preterite or imperfect verbs can be used in one sentence.
 

I got up at 7, showered, got dressed, ate breakfast and left the       house at 9. Yo me levanté a las 7, me duché, me vestí, desayuné, y salí de la       casa a las 9.  (Each action got completed before the next one began.)

The other day while I was washing the dishes, my husband       was repairing a lamp. El otro día mientras yo lavaba los platos, mi esposo reparaba una       lámpara. (Two actions were on-going at the same time. It’s not important       who got done first.)

“State” verbs (or verbs that represent a pre-existing condition)
There is a group of verbs that represents actions that do not generally have a clear beginning or end.  These verbs tend to be ones of being, feeling, seeming, wanting etc.  ALWAYS use the imperfect tense with the following verbs, unless a clear beginning or end is evident.