Hello Lyon'ka, 'a/an' means one of many so Berlin is 'an' interesting city because it is not the only interesting city - there are many others. Having introduced Berlin as an interesting city you could now use 'the' as we know that you are talking about 'the' interesting city called Berlin. Hope this helps Bob

We use 'a' or 'an' to refer to one of something, but not a specific one. Use 'the' to refer to a specific object, like "the Eifell tower", or "the Great pyramid". If I said "I live in a house", it just means I don't live in a flat or bungalow, but if I said "I live in the house on the corner with the red door", that is specific. There are many interesting cities, so each of them is "an interesting city". If I looked out of the window and saw a man walking a dog, that's quite vague, but if there were two men walking dogs, and I wanted to differentiate between them, I might say "The man walking the black labrador..." When referring to something unique, we always use "the" - e.g. "the leaning tower of Pisa". It is always specific. I hope this helps.