used to 表示过去常常做某事
He used to get up at six.
I used to be a teacher of English.
相当于情态动词,构成疑问或否定式 有两种情况,
eg Did he use to get up at six?
Used he to get up at six?
I didn't use to get up at six.
I usedn't to get up at six.
区别：be/get used to doing 这是习惯于做
He is used to getting up early.他习惯于早起.