The difference between USED TO and WOULD

Used to and would are both use to describe something that happened regularly in the past but doesn't happen any longer, as shown in the following two sentences about quitting smoking:

1. I used to smoke, but I quit last year. 
2. Whenever I craved a cigarette, I would chew gum instead. 

However, there are two important differences between used to and would. The first difference is that would should not be used unless it has already been established that the time frame is in the past, while used to does not require this. This example, with used to at the beginning, sounds natural:

• I used to watch cartoons every Saturday morning when I was very little. Now I rarely watch TV.

However, when used to is replaced with would, the same example becomes awkward and ungrammatical: 

• *I would watch cartoons every Saturday morning when I was very little. Now I rarely watch TV.

But if the past time frame is established before would appears, would sounds fine. 

• When I was little, I would get up and watch cartoons every Saturday morning. Now I rarely watch TV.

The second difference between used to and would is that would is not used with stative verbs such as love, be, understand, and feel. Compare these two sentences with the stative verb love:  

1. When I was a student, I used to love sleeping late on the weekends. (natural and grammatical)
2. *When I was a student, I would love sleeping late on weekends. (awkward and ungrammatical)

To summarize, the use of would is more restricted than that of used to. Therefore, if you aren't sure which one to use, it's best to choose used to.