Form:

• regular verbs (V+-d/-ed) / irregular verbs
  
• same form for all persons, except to be (was/were)
• auxiliary DID for negatives and questions

Use:

• definite past action or state (long or short)
• finished past actions

Form:

• WAS / WERE + VERB-ing

Use:

• to describe a past action at some point between its beginning and its end
  
• combined with a simple past action to indicate an interrupted action
• often used in a past simple narrative to describe the background history or environment