In the past progressive tense, the helping verbs are was and were. This tense is used to denote an action which is going on at some time in the past.

The time of action may or may not be indicated. It also shows that an action was begun and continued at a time in the past when another action took place.

• The girls were decorating the stage when I arrived.

• The girls were decorating the stage.

The negative form has the word not between the helping verb and the main verb + ing.

• The girls were not decorating the stage when I arrived.

• The girls were not decorating the stage.