Past ???

All the past tenses are used to express activities at a definite time in the past.

The past simple describes:
1. an activity at a definite time in the past
The study of airbags was started last year.
The past continuous describes:
1. an activity which is a time frame for another activity.
While we were studying the airbags, we made a significant discovery. While our team was studying performance, another team was looking at the characteristics.
The past perfect describes:
1. An activity that happened earlier than another activity in the past.
Our studies showed how well the equipment had performed.