I stayed in US for 5 months in 2011. I have submitted 1040EZ as that was the only one applicable in the H & R block that I had been to. Would there be a huge difference in % of tax return between the two?

Yes, there is a BIG difference.

If you were here for only five months, then by law you are a non-resident alien. Form 1040Nr or 1040NR-EZ are the ONLY forms you could legally file.

It is likely you cannot claim the standard deduction, but rather,you MUST itemize.

You will need to amend your return using Form 1040X and attaching a properly-prepared Form 1040NR-EZ with the word "AMENDED" printed on the top of the 1st page of the Form 1040NR-EZ.

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