"been" is the past participle form of the verb to be. So I would say that the main function is to form perfect tenses of the verb to BE. Within perfect tenses you can find present perfect (I have been ), present perfect continuous (I have been speaking), past perfect (I had been), past perfect continuous (I had been speaking) , Ex : more than 20 people had been killed .
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I have been working for 3 years means ,still you are working
2 I have been worked means you have worked earlier there not now
3 I have worked means .. Recently you have left the job..
Hope you have understood.
I have lived in this house for twenty years. (I still live in this house) Living continues
I have not seen Tom for ages. ( But I see him now.) Not continuing
1) War, fight or someone killed 20 people. ie 20 people died.
2) 20 people killed someone or group of people. This happened in the past to the past