The past continuous tense is most commonly used when
two past actions, one interrupting the other and one longer
than the other, happened at the same time. In this situation,
the simple past tense is used for the shorter action and the
past continuous tense is used for the longer action.

In such a situation, the connecting words while and when
are usually used in order to make the tense differences clear:
while is used to identify the longer action and when is often
used to identify the shorter one:

I was driving home. I ran out of gas.

Because "I was driving home" is the longer action and
"I ran out of gas" is the shorter one, while can be used
to introduce "I was driving home" and when can be used
to introduce "I ran out of gas":

While I was driving home, I ran out of gas. /
I ran out of gas while I was driving home.

When I ran out of gas, I was driving home. /
I was driving home when I ran out of gas.

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Jim was working in Chicago.
Jim bought his first car.

Because "Jim was working in Chicago" is the longer
action and "Jim bought his first car" is the shorter one,
while can be used to introduce "Jim was working in
Chicago" and when can be used to introduce "Jim bought
his first car":

While Jim was working in Chicago, he bought his
first car. / Jim bought his first car while he was
working in Chicago.

When Jim bought his first car, he was working in
Chicago. / Jim was working in Chicago when he
bought his first car.

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Sometimes two short actions happen at almost the same
time in the past. In this situation, use when before the
first action:

Julia fell down. Fred helped her get up. --->

When Julia fell down, Fred helped her get up. /
Fred helped Julia get up when she fell down.

Diego lost the race. Diego almost cried. --->

When Diego lost the race, he almost cried. /
Diego almost cried when he lost the race.

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Sometimes two long actions happen at the same time
in the past. In this situation, use while before the most
important of the two actions.

The chairman was speaking. Everyone was
listening intently. --->

While the chairman was speaking, everyone was
listening intently. / Everyone was listening intently
while the chairman was speaking.

Jane was planning her day. Jane was driving to work. --->

While Jane was driving to work, she was planning
her day. / Jane was planning her day while she was
driving to work.

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Special Note:

Notice that if a sentence begins with when or while,
a comma is used:

When I ran out of gas, . . . . .
When Jim bought his first car, . . . . .
When Julia fell down, . . . . .
When Diego almost lost the race, . . . . .

While I was driving home, . . . . .
While Jim was living in Chicago, . . . . .
While the chairman was speaking, . . . . .
While Jane was driving to work, . . . . .

Notice that if a sentence has when or while in the middle,
no comma is used:

I was driving home when . . . . .
Jim was living in Chicago when . . . . .
Julia hurt her ankle when . . . . .
Diego almost cried when . . . . .

I ran out of gas while . . . . .
Jim bought his first car while . . . . .
Everyone was listening intently while . . . . .
Jane was planning her day while . . . . .