Tax laws dictate that if you are legally married as of Dec. 31 of the tax year you are filing for then you need to either file a joint tax return together or file separate returns as "married filing separately". You're not allowed to file as single if you are legally married. There's also a rule that you either both have to take the standard deduction or both itemize.
If you file separately, some credits become disallowed. Filing jointly typically produces a better result, but with the Trump tax changes it's gotten a little more vague and sometimes filing separately is better. A lot of people who file separately choose to because of income based student loan repayments.
When we prep your return we automatically check both ways to determine the best result for you, but feel free to ask what the end result is of each if you are curious!
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