Sentence Correction

Past Progressive-A lot has been spoken about Present Progressive,but nothing much has been spoken about Past Progressive.

Past Progressive: Sandy was playing Soccer yesterday.

What is this supposed to mean.

In case of Past Progressive:Sandy is playing soccer.The present progressive indicates action happening right now,but whats not clear is what does the Past Progessive indicate?Yes I understand,that it happened in the past,but what does the to be verb+ '-ing' do to it additionally?Does it give it a time frame or something similar?Please clarify.

She was playing with her friends when the baby sitter arrived.

Whats the sequence of action in this?Did she just start playing,or was she already playing and continued on,or was it that she was already playing but stopped when the baby sitter arrived?What does the Past Progressive do here?

Now lets look at this one,both actions are in simple past here
She played with her friends when the baby sitter arrived.

According to Manhattan, in the above sentence the action played takes place after the babysitter arrived.How can we know this for sure?

Please explain the 'Past progressive' and 'when' concept.

Also please explain the difference between simple past and past progressive.I want to understand how they're different including the extended time frame the latter conveys.
Thanks

Thanks Ceilidh,
When you say ongoing action,does the action stop when the babysitter arrived?Or does it continue even after the babysitter arrived?Can you explain as to when the ongoing action started/ended?

ceilidh.erickson wrote:
In the example She was playing with her friends when the baby sitter arrived.
At the moment when the babysitter arrived, the playing was an ongoing action. It was still happening when the completed action "arrived" took place. The "when" tells us that the completed action interrupted the ongoing action.

Ok.Correct me if Im wrong.
Past progressive is an ongoing action that happened in the past,it started some time in the past.When we say ongoing,till when is it ongoing?Is it ongoing till a certain point in time?

ceilidh.erickson wrote:

When you say ongoing action,does the action stop when the babysitter arrived?Or does it continue even after the babysitter arrived?Can you explain as to when the ongoing action started/ended?

Actually, it's impossible to tell just from verb tense - both interpretations could be accurate. The rest of the sentence would have to give us more information:
She was playing with her friends when the baby sitter arrived, and she continued to play.
She was playing with her friends when the baby sitter arrived, but then she stopped.