The pooliceman was smoking on Sunday.
Imagine that you hade been with the policeman on Sunday and saw him smoking cigarettes and then you met someone who said that the policeman didn't smoke. The past progressive could be used to show that, while it may be genreally the case that ther policeman doesn't smoke, it is not alwatys true.
OK I have to rush- will be back later to look at the others.
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