SJ88“Does that mean that you can't use it when you are supposed to be using the past participle, or that you just use "used to" whether or not its in the past or past participle?
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"Used to" is verb idiom. That's all that matters. Situation have been described above. It is used as an auxiliary to express habitual or accustomed actions, states, etc., taking place in the past but not continuing into the present. (the citation).
The question you gave us is inexplicable. "Used to" and "used not to" are forms which are acceptable.