I used to go to the zoo when I was younger means that I went in the distant past and it is a way of expressing something that one has fallen out of the habit of doing. When a person wants to ask somebody a question about his old habits or customs he could use this form, as it is distinct from just using the simple past for any past event that was completed.
The key is using it as an affirmative answer and not as a negative answer to the question using it in the past. So for a question "Did you use to skate?" it is best to answer with "I did" or "I used to". So one can avoid saying something like "I did not use to…"Similarly if you have to answer in the negative, it is best to answer with the simple past. Note that with the past question there is no "d" on the "use", it is only present on the "use" in the answer: "Yes I used to."
The student can learn used to together with other similar grammatical forms like supposed to, in that both are used before an infinitive and they both refer to an alternative state if mind with reference to the present. The "used to" gets the listener to discover what someone's past habits were and "supposed to" is said when the person had to do something instead of something else: "What are you supposed to do if…"
Both forms can be used with the past of be as in "what were you used to doing" and this allows the questioner to find out information about what the subject did during an interval in the past. Supposed to can also be combined with the past continuous to create a statement like "what were you supposed to do" for something that someone had to do during a past period but did not fulfill that task. Answering the question in the past or the present with "when" or "if" helps create what we call a first conditional that will be discussed later.
There is a tendency to oversimplify the language by using the simple past everywhere but with the listener can understand what the speaker meant to say at various point in time.
