You're right. The first, with the past simple, is just a finished past action. You could continue that with something like...."I sat here yesterday with my friends, and then we walked into the centre". This indicates that the first action finished before the second one started. The second, with the past continuous, can be used alone. It's OK to say "I was sitting here yesterday with my friends", and nothing more. If you want to continue that, you could say something like..."I was sitting here yesterday with my friends when an egg hit me in the face" This indicates that the first action was still in progress when the second one happened. You can also use 'while', which indicates doing two things at the same time..."I was sitting here yesterday with my friends while eating an egg". Hope that helps! :)

Just a friendly recommendation: you are obviously well-skilled in English. However, please don't write "r" for "are" or use any of these other "words": u, ur, gonna, wanna. They give the impression that your English knowledge is not good.

you know a lot already and what you are saying is true. The past continuous in English is often a good match for the imperfect in other languages. The past continuous can be used to describe some action going on when then something else happens. I was in the room when I heard the gunshot, I was sitting in the room, I sat in the room.... The last sentence "I sat in the room when I heard the gunshot" is strange , you are correct, and typically you will not hear an educated native English speaker construct such a sentence.

Thank you very much, all of you. I read all answers and will remember them. It's a pleasure to improve my English with all of you and thanks again for all your advices. I really appreciate them.