This is a practical oversimplification which will be right most of the time. For more detailed information see Mastery of the Tenses!
For describing finished time
- I saw you yesterday, you looked very happy.
- I told you 2 minutes ago!
- She worked very hard last year and she passed all her exams.
For interrupting past actions.
- The phone rang while I was cooking.
- The cat ran across me as I was driving.
For Second Conditionals, IF clause
- If I were you, I would...
- If she had more time, she would...
- If they won the lottery, they would...