If you are legally married on December 31, 2018, then you will be required to file as married on your tax return. Your filing status options for 2018 will be married filing jointly or married filing separately.
Generally, filing jointly will give you a bigger refund or fewer taxes due. When you file separately, your tax rate is higher and you won't be able to claim:
On top of that, if you live in the community property states of Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington, or Wisconsin, you have to deal with community property allocations and adjustments, which adds extra work and complexity to your tax preparation chores.
Tip: Only taxpayers who were still legally married as of December 31, 2018, are able to file as married, whether jointly or separately.
Filing jointly means you file one tax return. When filing separately, you file two tax returns.