Elderly retired Resident Alien in FL

[AliStraz]

My father is a 76 year old Resident Alien partially living in FL but has never worked in the U.S.   His only income is pension from his country of origin.   What Federal tax form does he need to fill out?  Can it be backdated for a number of years?   What sort of "proof" would he need to provide?  Is there any other documentation that will be needed to file taxes to establish full time residency in FL? 

[guywong]

You may want to see if there is a tax treaty between the United States and the country your father came from.  If there is one, the tax treaty may tell you if the pension from the old country is taxable or not.  If no tax treaty, my off the top answer is to report the pension checks received on the "Other Income" line.  If there is an income tax paid to the country of origin on the pension, then the foreign tax paid may be used to offset all or part of the U.S. income tax., on a Form 1116.

As a general rule, 183 days of stay establishes residency or domicile in a state.