If I lived in non-resident state on December 31, do I enter this state, or do I enter my legal resident state (even though I wasn't living there on 12/31)?

You enter you legal residence state. When asked if you live in another state, answer no (you were only a temporary resident, which makes you a non resident for tax purposes).

When asked if you earned money in another state, answer yes if you made money while a temporary resident.

That answer is based on certain assumptions. Primarily that your stay in the non resident state has a  temporary purpose and your intent is to return to your home state, in the near future. Otherwise, you are a part year resident of both states.

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If you lived in the state you worked in for 30 days or more you enter this state on your tax forms.

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