"Used to" is a structure used to talk about past habits or states that were true in the past but are not true anymore. It indicates that something was regular or habitual in the past but is no longer the case. For example, "I used to play the piano when I was younger." To use it, simply place "used to" followed by the base form of the verb. Note that "used to" is not used in the present or future tenses; only for past habits or states.