When you say: "I was in Paris" it could be for a short time and it was finished.But if you say : " I have been in Paris" it is for a long time that you were there and it has not finished yet,and it continues till present.

Hello :) It's confusing because information is missing. "I was in Paris" ...when? "I was in Paris two years ago" is a correct sentence. Simple past needs a time when it happened, otherwise the phrase is meaningless. "I have been in Paris." This is a complete sentence. We use present perfect to talk about now (using past events), and that includes talking about your experience; the fact you're telling us about it is relevant now. "I had been in Paris" ...another phrase which is missing information. Past perfect is used to explain or give some back-story to an event in simple past (these tenses work together). The past perfect grammar tells the listener/reader that this event happened earlier. "When I met my wife in Pigalle, I had already been in Paris for two years." It's a very neat way to break up a linear story.