According to the North Dakota instructions:
A North Dakota Resident is an individual that is domiciled in North Dakota for the tax year. If you are not domiciled in North Dakota but maintain a permanent place of abode in North Dakota and spend more than 210 days in the state, you must file as a resident.
A Nonresident of North Dakota is an individual that was not a resident of North Dakota and did not spend more than 210 days in the state during the tax year.
A Part-Year Resident is an individual that moved into or out of the state during the tax year.
If one spouse is a resident and the other is not, file a nonresident joint return and complete Schedule ND-1NR indicating the residency status of each spouse.
For more information: