Thank you for using justanswer. You may not file your tax returns together in the sense that you may not combine the income you received in 2006 with that you received in 2007 and report it all on your 2007 tax return. You will have to file each year individually, but you are well within the 3 year window that allows you to retain your right to any refunds that might be due to you. If you owed for 2006, then you will certainly want to file that one ASAP as it is already accruing interest and penalty. Please see the link below for more in depth information
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Help Yourself by Filing Past-Due Tax Returns
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Yes, you may always file a tax return, even if its only to claim federal income tax withheld. If there was nothing withheld, and if all of your money for 2006 was earned while you were incarcerated (making you ineligible for any earned income credit), then although you can file a tax return for 2006, there might not be any benefit to you for doing so.
You can find the tax forms for tax year 2006 here:
2006 Forms, Instructions, and Publications
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