The simple past tense is used to describe a completed action that happened at a specific time in the past, while the past continuous tense describes an action that was ongoing or in progress at, before, or after another action in the past. For example, "I ate dinner at 7pm" uses the simple past tense to indicate a single completed action, whereas "I was watching TV while you were cooking dinner" uses the past continuous tense to describe two simultaneous ongoing actions.