use if the sentence is a single, completed action, and if the sentence passes this test:
1)The verb expresses an action
2)If yes, then ask:
A)Is the action beginning?
B)Has the action ended?
C)Is the reference to the action a single occurrence?
use if the verb is not an action verb, if the verb is a repeated action, or if the verb is a background action; "used to" indicates a repeated action; use with simultaneous action; use with age, time, weather, stories, thinking verbs, emotion