1. He used to take a walk in the park, didn't he?
2. Didn't he used to take a walk?
3. Didn't he use to take a walk?
4. He used to take a walk in the park, usedn't he?
5. He used to take a walk in the park, usen't he?
6. Usedn't he to take a walk?
7. Usen't he to take a walk?
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They seem to be a little complicated? Which ones are grammatical? Which ones are not?
1 and 2 are correct. The others are not.
You can see the difference between 2 (correct) and 3 (incorrect), right?
In 4-7, "usedn't" is incorrect. That's not a tag line used in English.