used to = 以前 It is the past tense of the present simple. Present simple = repeated actions, or states, spanning the present used to = repeated actions, or states, spanning a finished past time frame I used to live in a flat, now I live in a house. In this sentence, there is no adverb or expression of a definite time. I was living in a flat then. Now I'm living in a house. In this sentence we have an adverb or expression of time. The difference is sometimes very small. But note, do not use "used to" when you have an expression of a definite time. If you want to express a repeated action in the past at a definite time, use "would". When I was a boy, we WOULD go to the park a lot and play football. This is one of the trickier rules of English. It takes a bit of practice to get it down pat.

Well, you can use both, but there will be a change in the meaning. For example: I used to live in Canada, but now I live in Germany. ..............A........................................B In this kind of sentence there is part A and part B. Part A introduces a change in the cicumstances, like in the previous example. It may also be used to express cause - effect. For example. I used to live in Italy for 5 years, so I can speak Italian. .........................A............................. B In the other structure you asked about "I was living..." you find different type of elements. In this case you find sentences like. I was living in Africa when my father won the price. The first sentences is niether a cause nor a change of circumtances related to the second sentece, it just present parallel situations to an unrelated event. I hope this helps. Chees, Mario

They are pretty much equivalent, but I would moreso expect to see 'were living' used in the context of a longer story.