Married both over 65, one still working other SS benefits under $12,000. Can file married separately? the one income under $12000 does not file correct?

[macuser_22 · Accepted solution]

"...the one income under $12000 does not file correct?..."

No,  Not correct.

MFS must file a tax return if gross income was more than $5.00. 

https://www.irs.gov/pub/irs-pdf/p17.pdf  page 6

If you file separately and you lived with your spouse anytime during 2018 then up to 85% of Social Security can be taxable. 
https://www.irs.gov/pub/irs-pdf/p17.pdf  page 21.

[SweetieJean]

The IRS has rules in place that cover this. If your file MFS, you must declare your Soc Sec on the MFS return. There are also special rules if you are in a Community Property State.