Past continuous

Past continuous
 we form the past continuous with subject+was/were + verb(ing)
Use
we use the past continuous to talk about something which was in progress at a past time.The action or situation had started but it had not finished a that time.

     More example
        I saw you last night.You were waiting for a bus.
        Was Sue working at 10  o'clock yesterday morning.

compare the user of the past continuous and past simple
                       past continuous                                                                past simple      

I was writing a letter (=I was in the middle of writing it)

I wrote a letter(=I started and finished)

-> the past continuous describes a longer action or situation which was in progress at a past time: the past simple describes a shorter action which happened in the middle of the longer action or interrupt it.         

But to say that one thing happened after another .we can use the past simple.
when the telephone rang.Kate answered it.
We sheltered under a tree when it started to rain.
compare:

When Kate came home Ken was making some tea.(Ken was in the middle of making some tea. Then Kate came home.)

When  Kate came home, Ken made some tea. (Kate came home.Then Ken made some tea.)

-> We often use the past continuous and past simple together in a sentence.Notice what happens in sentence like these:
              Past continuous                                                                         Past simple                               

I was driving along when Kate was watching TV We were walking in the park

When suddenly a child ran across the road. The telephone rang. When it started to rain.