(1) 在表示过去重复的习惯时，used to可用would代替。
When we were boys we used to/would go swimming every summer.
小时候，每到夏天我们都要去游泳。
He used to/would spend every penny he earned on books．
过去，他通常把挣来的钱全花在买书上。
(2) used to 强调过去的行为同现在的对比，“过去怎样，则现在却不这样了。”would则表示单纯的过去习惯，常与often, every day 等连用。
He would take a walk near the forest in the evening.
以前他晚间常在森林附近散步。
He used to take a walk near the forest in the evening.
(而现在已不在那里散步了。)
He used to be a very strong man.
过去很强壮，而现在体质差。
We would sit in the yard every evening and listen to his story.(过去重复的行为)
(3)表示过去的状态，只用used to,不用would替换。
There used to be to park here.以前这儿有一个公园。
There used to be a small village here.这里曾经有一个小村庄.