Past Progressive vs. Past Perf Prog. Usage

[s.t. bischoff]

Hello all,

I'm wondering if anyone can direct me to an account of the different uses
of the past progressive (1) and past perfective progressive (2).

(1) In 1991, I was living in London.
(2) In 1991, I had been living in London.

In particular, I'm looking for a means of explaining the difference,in
usage, to non-native speakers of English.

Thanks,
Shannon