Mistake on previous tax returns, please help

I think I am in a bit of a pickle here. After spending all morning looking at some of these tax questions here on the forum, I realise that I may have been filing the wrong status. For the years of 2015 - 2018 I have filed 'Single'. I assumed that is what I was supposed to file because I am married to a UK citizen, he has no ties to the US, and we don't have a joint bank account. Our finances are totally separate, my income is primarily used to buy groceries and pay a couple of small bills.

After reading here, I have seen some say that I am supposed to be filing 'Married Filing Separately'. My husband argues that this status implies that we are married but that we are filing separate returns, and that would be wrong, as he isn't filing a US tax return because he is a UK citizen.
I cannot wrap my head around this at all, and reading any kind of tax guidance does not help me, because its too complicated, I don't understand enough for it to make sense. I have always used software to do my taxes.
I have read that if I am supposed to be filing 'married filing separately', then I will need to file amended returns ASAP. I have no idea how to do this. Can anyone please tell me how to go about doing this, or point me in the direction of a resource to get info about how to do this?

Just FYI, my income is quite small, as I work part time. I don't have any savings, isa, and not much of a UK pension at all. There is not anything complicated at all about my return, other than the fact I am a married US citizen living and working abroad. I can't really afford to pay hundreds of pounds to fix this, so any guidance would be helpful.

Thanks in advance for any advice.

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You should definitely have been filing Married Filing Separately.  I think you can "simply" file amended returns using form 1040X. The IRS website below says you have 3 years in which to file an amended return so returns '16, '17 and '18 should be okay. Not sure about 2015.

https://www.irs.gov/newsroom/amending-your-tax-return-ten-tips

If you don't owe any tax on your amended returns then maybe don't even bother filing an amended return for 2015. The chances of the IRS auditing your 2015 return is going to be next to zero since you don't owe any tax.

https://www.forbes.com/sites/robertwood/2019/02/13/amending-your-taxes-be-careful-with-irs/#7d9cd2e23b93

Normally the IRS has three years to audit a tax return. You might assume that filing an amended tax return would restart that three-year statute of limitations. Surprisingly, it doesn’t. In fact, if your amended return shows an increase in tax, and you submit the amended return within 60 days before the three-year statute runs, the IRS has only 60 days after it receives the amended return to make an assessment. This narrow window can present planning opportunities. Some people amend a return right before the statute expires. Plus, note that an amended return that does not report a net increase in tax does not trigger any extension of the statute of limitations.

You should definitely have been filing Married Filing Separately.  I think you can "simply" file amended returns using form 1040X. The IRS website below says you have 3 years in which to file an amended return so returns '16, '17 and '18 should be okay. Not sure about 2015.

https://www.irs.gov/newsroom/amending-your-tax-return-ten-tips

Thanks for that info, sounds simple enough. So sounds like all I would need to change is the filing status, I havent owed any tax in any of those years. Can I send all three amendments at once?

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Thanks for that info, sounds simple enough. So sounds like all I would need to change is the filing status, I havent owed any tax in any of those years. Can I send all three amendments at once?

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Ah I just found the answer to that question, thanks so much for your help.

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