Past Progressive

(g) I was walking down the street when it began to rain.

(h) While I was walking down the street, it began to rain.

(i) Rita was standing under a tree when it began to rain.

(j) At eight o'clock last night, I was studying.

(k) Last year at this time, I was attending school.

In (g):1st: I was walking down the street.
2nd: It began to rain.

Both actions occurred at the same time, but one action began earlier and was in progress when the other action occurred.
In (j): My studying began before 8:00, was in progress at that time, and probably continued.

(l) While I was studying in one room of our apartment, my roomate was having a party in the other room.

Sometimes the past progressive is used in both parts of a sentence when two actions are in progress simultaneously.