There is a simple answer to this question, rhapsodyofwords. The two forms are interchangeable when you are talking about a repeated action in the past which no longer occurs: When I was a child, I would walk for hours in the woods. When I was a child, I used to walk for hours in the woods. However, to describe a state/situation which was true in the past but is no longer the case, you can only use 'used to': I used to live in the countryside. You can't use 'would' in this situation.

I think you are referring to the following kind of expressions: "I would often go swimming" "I often used to go swimming." There is probably a formal grammatical explanation for this. However, my own instinctive sense is that their meanings are the same and the difference in their usage is more a question of style. For example, you could say this: "I used to go shopping in street markets. I would often buy fruit and vegetables. Then I would stop for a coffee with a a friend etc" For the first expression, you could say "would" but you would have to add "an expression of time such as "in the past" to make the meaning clear. In the second and third expressions, if you said "used to" instead of would, this would sound a bit unnatural. So, you could say that "used to" sounds more natural when relating to a general comment about the past. "Would" sounds more natural to express more particular activities in the past, to help bring the listener back to that time with you. I hope that makes sense - it's not a classical explanation and it may not be comprehensive but I hope it helps. Michael

They're completely different words, and have no relationship to each other. "Used to" is a phrasal verb that means either something that happened regularly in the past ("I used to live in Berlin") or something you're accustomed to ("I am used to sleeping in hotel beds because I travel a lot"). "Would" is a modal verb that means either the past tense of "will" ("He said he would be busy for the next four hours"); or else is used in polite requests ("Would you close the window, please?); or else is used in conditional sentences ("If we cut our prices we would probably sell more units").