Thank you kindly, Slowtraveler
The tax year doesn't change for expats. Whenever you file your 2019 tax return, it will be for Jan1 - Dec 31.
If you are not married by Dec 31, 2019, you are Single for tax purposes.
If you leave the US in June 2019, finish a year abroad in June 2020, file your tax return in Oct 2020, you will get a partial foreign income exclusion for June - Dec 2019. Any taxes owed would still be due in April 2020.