Past Continuous

1. A longer action in the past was interrupted

1.1. The interruption is usually a shorter action in the Simple Past.

1.1.1. I was watching TV when she called.

1.1.2. When the phone rang, she was writing a letter.

1.1.3. While we were having the picnic, it started to rain.

2. Specific Time as an Interruption

2.1. Last night at 6 PM, I was eating dinner.

2.1.1. At midnight, we were still driving through the desert.

3. Parallel Actions

3.1. I was studying while he was making dinner.

4. Repetition and Irritation with "Always"

4.1. She was always coming to class late