The phrase "used to" is part of a group of phrases that are misused in the English language a lot. For example, many people don't speak it correctly because they drop the 'd' when they pronounce it. Hence, children and second language learners have a difficult time knowing what the correct usage is. Other misused phrases are "supposed to," "could have," and "supposedly."
1. I used to be good at math.
2. I used to collect stamps.
3. I used to watch television.
4. He used to cook for his family.
These are good sentences to practice in your daily speaking so that you will remember to write them correctly later.