Who is an Illinois resident?
You are an Illinois resident if you were domiciled in Illinois for the entire tax year. Your domicile is the place where you reside and the place where you intend to return after temporary absences. Temporary absences may include duty in the U.S. Armed Forces, residence in a foreign country, out-of-state residence as a student, or out-of-state residence during the winter or summer. If you are absent from Illinois for one year or more, we will presume you are a nonresident of Illinois.
If you filed a joint federal return and one spouse is an Illinois resident while the other spouse is a nonresident or a part-year resident, you may file separate Illinois returns. If you file a joint Illinois return, you will both be taxed as residents.