1) 1665,Isaak Newton made an in important discovery
We use the Past Simple (Indefinite) for comleted actions in the past
2)He was studying at Cambtige University
We use the Past Continuous (Progressive) to describe the situation at a particular time in the past
3)He was sitting under a tree when an apple fell on his head
We use the Past Continuous (Progressive) and Past Simple (Indefinite) together when one action in progress was interrupted by another action.