Past simple vs past continuous - what's the difference?

Past simple - a completed action at a specific time:
"I called her at 8pm."
"We finished the project."
"She left the company last year."

Past continuous - an ongoing action in the past, often interrupted:
"I was calling her when the power cut out."
"We were working on it all morning."
"She was leaving when he arrived."

Past continuous (background) + past simple (interruption):
"I was having a shower when the phone rang."
"They were eating dinner when the fire alarm went off."
The continuous = the longer, ongoing action. The simple = the interruption.

Two things happening at the same time - use continuous for both:
"While I was cooking, she was setting the table."

Completed actions at a set time:
"I was going to the shop yesterday." ✗ (sounds unfinished)
"I went to the shop yesterday." ✓

If you can imagine the scene still playing in the background, use past continuous. If the action is complete and done, use past simple.