Hi, Jakub! First of all, the sentences should be as follows so they sound natural: I was having dinner. I have been having dinner. In the first case, you're done eating since you're talking about an incident in the past. It seems to be the answer to 'What were you doing?' In the second, one explanation is that you are still having dinner. It could be an answer to 'What have you been doing (all this time)?' If you extended the sentence to include a place, though--as in 'I've been having dinner at Burger King.'--you could be telling someone about what you usually do and is not related to a specific incident. You could be answering the question, 'Where have you been having dinner?', for example. Hope this helps! Regards, ServiceTier

I was eating when I met Sally. I've been eating a dinner from 3 hours and I'm still hungry.