1. They used to live there, didn't they?
2. They used to live there, usedn't they?
(Are both OK?)
3. Didn't they use to live there?
4. Usedn't they to live there?
5. Did they not used to live there?
6. Did they use to live there?
7. Did they used to live there?
(Which ones are grammatical?)
1. OK
2. not OK
3. not OK
4. not OK
5. OK
6. not OK
7. OK