Preterite or Imperfect? What rule?

Last night, Maria made pizza. PRETERITE: one time action

Every Friday I bought a new CD. IMPERFECT: habitual action.

My family and I cooked, ate, cleaned, and went to the park. PRETERITE: a list of actions

Once I ate a worm. PRETERITE: one time action

The students finished the project. PRETERITE: beginning or end of action

When Eva was 13 years old, she ice skated one time. IMPERFECT: age in the past PRETERITE: one time action

While David walked, he looked at his watch. IMPERFECT: continuous action

At 7:30, Raul cried because his girlfriend broke-up with him. PRETERITE: one time action, beginning or end of action

School was always a fun place for Jimmy. IMPERFECT: setting the stage or background.

I studied everyday. IMPERFECT: habitual or continuous action

Jane was sad and depressed because she realized that her boyfriend was fat. IMPERFECT: setting the stage PRETERITE: interrupt action, one time action IMPERFECT: setting the stage, description

It was 8:52 when Roger began to write his paper. IMPERFECT: time in the past PRETERITE: beginning or end of action

John was taking his test when suddenly the fire alarm went off. IMPERFECT: continuing action in the past PRETERITE: interruption