used to

Từ điển Oxford Advanced Learner 8th

used to [used to] BrE [ˈjuːst tə] NAmE [ˈjuːst tə] before vowels and finally [ˈjuːst tu] NAmE [ˈjuːst tu] modal verb (negative didn't use toBrE [dɪdnt ˈjuːs tə] ; NAmE [dɪdnt ˈjuːs tə] before vowels and finally [dɪdnt ˈjuːs tu] ; NAmE [dɪdnt ˈjuːs tu] (BrE also, old-fashioned or formal used not toshort form usedn't toBrE [ˈjuːsnt tə] ; NAmE [ˈjuːsnt tə] before vowels and finally [ˈjuːsnt tu] ; NAmE [ˈjuːsnt tu] )
used to say that sth happened continuously or frequently during a period in the past
• I used to live in London.
• We used to go sailing on the lake in summer.
•I didn't use to like him much when we were at school.
•You used to see a lot of her, didn't you?

Grammar Point:
modal verbs
The modal verbs are can, could, may, might, must, ought to, shall, should, will and would. Dare, need, have to and used to also share some of the features of modal verbs.
Modal verbs have only one form. They have no -ing or -ed forms and do not add -s to the 3rd person singular form: ▪ He can speak three languages. ◇ ▪ She will try and visit tomorrow.
Modal verbs are followed by the infinitive of another verb without to. The exceptions are ought to and used to: ▪ You must find a job. ◇ ▪ You ought to stop smoking. ◇ ▪ I used to smoke but I gave up two years ago.
Questions are formed without do/does in the present, or did in the past: ▪ Can I invite Mary? ◇ ▪ Should I have invited Mary?
Negative sentences are formed with not or the short form -n’t and do not use do/does or did.
You will find more help with how to use modal verbs at the dictionary entries for each verb.

Grammar Point:
used to
Except in negatives and questions, the correct form is used to: ▪ I used to go there every Saturday. ◇ I use to go there every Saturday.
To form questions, use did: ▪ Did she use to have long hair? Note that the correct spelling is use to, not ‘used to’.
The negative form is usually didn’t use to, but in BrE this is quite informal and is not usually used in writing.
The negative form used not to (rather formal) and the question form used you to…? (old-fashioned and very formal) are only used in BrE, usually in writing.

Which Word?:
used to / be used to
Do not confuse used to do sth with be used to sth.
You use used to do sth to talk about something that happened regularly or was the case in the past, but is not now: ▪ I used to smoke, but I gave up a couple of years ago.
You use be used to sth/to doing sth to talk about something that you are familiar with so that it no longer seems new or strange to you: ▪ We’re used to the noise from the traffic now. ◇ ▪ I’m used to getting up early. You can also use get used to sth: ▪ Don’t worry — you’ll soon get used to his sense of humour. ◇ ▪ I didn’t think I could ever get used to living in a big city after living in the country.