This is a very common question among Spanish learners.

Keep three things in mind:

1. ALL of these words are about events that occurred in the PAST tense (do not confuse it with future tense nor present conditionals or it will drive you bonkers).

2. Imperfectos are ongoing, and the equivalents to the phrase "I was doing", "I was eating", or "I was walking".

3. Pretéritos are sudden, interrupted, and not ongoing. They are conjugated differently according to the action that caused them.

Yet, each has its own form and context:

Let's begin with imperfecto, which actually translates better into English.

The imperfecto conjugation is denoted for its "aba/iba" ending, which include the past tense words:

• iba (I was going)
• llevaba (I was taking)
• estaba (I was)
• buscaba (I was searching)
• miraba (I was staring/looking)
• compraba (I was buying)
• terminaba  (I was finishing)
• acababa (I was ending)

And then the "ía" ending in words such as

• tenía (as in age or possession) I was 18
• vestía (I was dressed)
• comía (I was eating) and
• dormía (I was sleeping) among many more.

If it is something done normally and consistently, you use imperfecto. You also use imperfectos to place situations "estaba", "vestia", "llevaba". Even if you were to tell time you use imperfectos: "Eran las 8";  "Eran los mejores tiempos, eran los peores tiempos."

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Pretérito is another story. The words that name actions which are not consistently done, or refer to events that are unique or interrupted, change drastically in grammatical structure.

For the word "Tener" (to have), the imperfecto tenía would change to "tuve" if the story that you are telling, or the event that you are narrating, speaks of one specific moment in time when something of particular importance caused you or someone to feel hunger.

Tenía hambre= I was hungry / Tuve hambre= I was/became hungry when (preterito)

Another list of preteritos includes all the words that, when conjugated, end in é, and í.

Leí (I read)- hablé ( spoke/talked) -comí (I ate)- viví (I lived)- traté (I tried)- llamé (I called)- conseguí (I got).

This being said, essentially the PRETERITO refers to actions in the past that occurred for a brief period of time, for a limited time, or just one or a few times.

The IMPERFECTO talks about actions in the past that were ongoing and consistent.