Used to is a grammatical structure that expresses a repeated past habit or action that no longer happens or is true. It is followed by the infinitive form of a verb. Examples provided demonstrate using used to with subjects like "I", "she", and proper nouns, followed by past actions like "sleep with her teddy bear", "play football", "wear glasses", and "eat a lot" to convey something that was regularly done in the past but not anymore. Activities are included to practice the use of used to.