“Could” questions are a type of grammatical construction in the English language that are used to ask about someone’s ability or possibility to do something in the past. They are formed by using the modal verb “could” and adding the subject of the sentence and the main verb. For example: “Could you help me yesterday?”, “Could I borrow your book?”, “Could they play the piano before?”. These questions are asking about someone’s ability or possibility to do something in the past. Could is used to talk about a past ability, or to be more polite in a request than “can”.