• Past Continuous + When 

• 
• Use the past continuous when one action is happening and another action interrupts it.  The interrupting action uses simple past.  Use when to link the past continuous action and past simple actions.  

  
  

  past continuous + when + simple past
  

  
  

  I was watching fireworks when my phone rang.

  (The phone interrupted the action of watching fireworks.)
•  Click on the play button to hear the audio

• He was walking to work when he fell.	When he fell, he was hanging holiday decorations.*
  What were you doing when I called?	I was making dinner when you called.

• *Remember! Use a comma (,) if the simple past action with when comes before the past continuous action.

 Source : Alison