Explanation of the dual status and first year choice
There seems to be a lot of confusion out there about the situation regarding residency, non-residency, dual status and first year choice. I would like to address this situation. Below I have outlined the facts AND the sources of support.
1. Residency status
There are two tests: the green card test and the substantial presence test.
If you DO satisfy either test then you are a resident. But you may have been a non-resident for part of the year and a resident for the other part. In other words you were a dual status alien. Your status CHANGED during the year.
If you DON'T satisfy either of these in 2005 then you are a non-resident. You are a non-resident for the WHOLE year unless you make a first year election.
This is all supported by IRC Section 7701(b)
2. First year election
If you are not a resident in 2005 because you don't pass either test but you are a resident in 2006 because you pass the substantial presence test in that year you can choose to be a resident for PART of 2005. IRC Section 7701(b)(4)
Why? Because if you think about it, you really began your residency in 2005 but it took some time to meet the substantial presence test as that test involves counting your days.
Which part of 2005 counts as residency? Basically you need to have 31 consecutive days in the US in 2005 (not counting days for which you were an exempt individual -- such as a student) and you need to have been in the US most of the time following the end of that 31 day period (in other words 31 days here in October and then disappearing for November and December won't work).
Your residency is going to start at the beginning of that 31 day period. IRC Section 7701(b)(4)(C)
Note, in determining the starting point DON'T count days that you were an exempt individual. IRC Section 7701(b)(4)(D).
So you are going to be a DUAL STATUS person. You will be a NON-RESIDENT for 2005 up to the start of the 31 days and a RESIDENT for the 31 days onward to the end of 2005.
3. First year choice and MARRIED to a resident at end of year
YOU ARE NOT A RESIDENT FOR THE FULL YEAR OF 2005 JUST BECAUSE YOU MADE THE FIRST YEAR ELECTION UNLESS YOU ARE MARRIED AND MAKE AN ADDITIONAL ELECTION.
If you are married to a resident alien or a US citizen at the end of the year THEN you may elect to be a resident for the ENTIRE year. If you are single this is not an option that is available to you.
IRC Section 6013(h).
IMPORTANT CAVEAT
I have simplified things as much as possible to make this clear. There are of course other issues to consider in each specific case and exceptions and qualifications to the above information. This should be a useful starting point for you but you should review the specifics of your case before taking any action.