I was cooking when the phone rang: when and while

A long action and a short one: when

This is the past continuous's big job. The long action takes the past continuous. The short action that cuts into it takes the past simple.

Either half can come first, with the same meaning:

While: two long actions at once

When both actions were long, both take the past continuous. While joins them:

• I was reading while Tom was cooking.
• The children were playing while their mother was working.

While usually goes with the long action, and when with the short one:

While she arrived sounds wrong: arriving takes a second, so there is nothing to be in the middle of.

Past continuous or past simple?

The past simple is the whole action, from start to end (the whole thing). The past continuous is the middle of it.

I read the letter means I got to the end. I was reading the letter does not say if I finished: usually, something else happened.

For a list of events, one after the other, use the past simple: I got up, made coffee and read the news. The past continuous cannot move a story on, because nothing in it finishes.

One past event before another needs the past perfect: When I arrived, the film had started. All the tenses side by side are on the English tenses page.

Verbs that do not take -ing

Some verbs are states, not actions, and you cannot be in the middle of a state. They stay in the past simple:

Some have an action meaning too, and then -ing is fine:

• I was thinking about you. (thinking as an action in your head; for an opinion: I thought it was Tuesday.)
• She was having lunch. (have = eat)

Everyday speech, especially British, also uses I was wanting to ask you something. as a polite set phrase. The whole list is on the stative verbs page.