1. Use for single events, past events, or a specific moment in time.
2. Use for a list of sequential actions.
3. Use for actions that interrupt the ongoing action.
4. Use for verbs referring to the senses.
When to use imperfect
1. Actions in progress in the past.
2. Habitual actions
3. Interrupted action
4. Time, weather, age, descriptions, and physical/mental state