All American citizens are liable for income tax. It doesn't matter whether you were born here or took your citizenship oath last week: once you start earning money as an American, you pay American taxes. In some cases, you may have to pay the IRS even if you're not a citizen yet.
It's not just new citizens the IRS expects money from. If you have a green card, making you a permanent foreign resident, you're not a citizen. The IRS, however, considers you a "tax resident" and expects you to pay on what you earn. There's little difference between the tax rules for resident aliens and the taxes for a full citizen. You have the same tax rates, filing statuses and deductions available as you do as a US citizen.
If you still have a job or investments in your home country, you pay American income tax on them once you become a citizen or tax resident. All the income you earn as a citizen, here and overseas, is subject to income tax. If you pay overseas as well, you can claim a tax credit in the United States to avoid paying income tax twice. You usually use Form 1116 to claim the tax credit and cut your American tax bill.
You have to have either a Social Security number or a taxpayer identification number to pay your taxes. If you work in the United States, a Social Security number proves your identity to your employers. If you have foreign income to report but don't have a Social Security card, you can file a W7 form with the IRS to have the agency issue you a TIN.
Once you become a citizen, you stay one, whether you remain in the United States or head home to another country. Because your income anywhere in the world is subject to American income tax, it doesn't matter where you live. As a US citizen living abroad, you do get an automatic two-month extension, so your deadline for filing is mid-June. If you ask for an extension, though, you only get one through October -- six months after the April deadline -- and not December.
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