The past progressive tense, also called the past continuous tense, is formed by using the helping (auxiliary) verb of “was” or “were” followed by the present participle form of the verb (ending in “ing”). This tense shows an action that happened at a specific moment in the past; the moment started prior to that moment but is not necessarily complete at the specific moment to which you refer. Sometimes the past progressive tense is used in conjunction with the simple past tense to show that a short action (simple past) occurred in the middle of a longer action (past progressive). Keep in mind that the difference between long and short actions is relative to the measurements of time that apply. For example, minutes could refer to a long action during which something occurred in seconds. “When” is often used to connect the two sentences. Consider the following examples:
Example 1: It was snowing all night (it is unclear when it started snowing, but it started snowing before the moment to which is referred, and it has not stopped snowing at that moment).
Example 2: The raindrops were pounding on the roof.
Example 3: It was raining when they walked to the store (“was raining” is in the past progressive tense, and “walked” is in the simple past tense – the long action is raining, and the short action is the walk to the store).