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. The
connecting words while and when are usually used in order
to make the tense differences clear:

I was taking a shower when you called me. /
When you called me, I was taking a shower.

_______________________

Bob and Carol were living in Chicago when
their first child was born. /

When Bob and Carol's first child was born,
they were living in Chicago.

_______________________

Julia was studying in California when she
met her husband. /

When Julia met her husband, she was studying
in California.

_______________________

Someone called me while I was taking a shower. /
While I was taking a shower, someone called me.

_______________________

Bob and Carol's first child was born
while they were living in Chicago.

While Bob and Carol were living in Chicago,
their first child was born.

_______________________

Julia met her husband while she was studying
in California. /

While Julia was studying in California,
she met her husband.

_____________________________________________

Special Notes: