 Use 2. Use 2 is a formal auxiliary. It is used in grammar in English in order to express a habit that was done in the past but is not continuing now. It expresses a discontinued habit. That means some action was being done in the past for some time but now it has been discontinued. So, by using the auxiliary used 2, it is very clear that whatever is being mentioned, the action or the action verb which is in the sentence would be expressing something that was done in the past and now it is no more there. For example, I used to live there when I was a boy. So, I was living there sometimes ago but now I am not living there. I was living there when I was a boy. So, in order to express that, that I am no more living there, it is useful to use used 2 in the sentence because then it will make the meaning very clear that this is no more the situation. I used to play badminton when I was young. Again it means that badminton was being played in the past. I was playing this game but now I have discontinued playing this game. It is not there anymore that I am playing this game. I used to play it in the past when I was young. In the question form, did you use to swim or did you use to study hard? That means some question is being asked about a habit which was happening in the past. Did you use to swim? That means is it so that in the past you were swimming or is it so that the past situation was that you were swimming but now you are no more swimming. So, it will be did you use to swim. So, by using the auxiliary used 2, time frame would come in the mind of the listener immediately. Immediately the listener would come to know that this is being talked about the past and now this habit is no more there. It has been discontinued. She used to visit her mother last year. Again she used to visit her mother sometimes ago and that was last year. Now she is not visiting her mother anymore. It was a habit in the past a year ago but now it has been discontinued for any reasons. But because we are using used 2 in the sentence it is very clear that the speaker means that the habit of visiting the mother has been discontinued and therefore there is no more visiting habit to the mother anymore. It just took place last year and that is over now. Now the situation is different. It is being mentioned about a habit which discontinued which was there in the past but now it is not there. So, with the help of the auxiliary verb it is important that we are able to express the time frame. When was it happening? It was happening sometimes ago and now it has been discontinued and the habit is not really there anymore. But it is being mentioned as a past habit which did continue for some time and then it discontinued and therefore the auxiliary used 2 is being used in order to express that something was happening in the past which is not continuing anymore as a habit.