Do I have to file for federal income tax for 2018?

If it was all on a W-2 with social security and medicare taken out no you don't have to file, otherwise yes you do.

I would... to get the EIC credit. Earned Income Credit. You had less than $15K in income and should be able to qualify for this credit even as a single person with no children.

and can receive $529

W-2, no--unless you had subsidized Obamacare.

Self employement, yes as threshold is $400.

Only if federal income tax was withheld from your pay and you want it refunded to you. You must file to claim that refund.

Not if that was your whole income.. however, you would get back anything they deducted from your check for Federal Withholding taxes as a refund if you do.

If you had any tax withheld and want it back you have to file.

You are not required to file.

If you are receiving a subsidy for your health insurance you still need to file a Federal tax return. If not then no, you don't because you have not reached the taxable minimum earnings for the year. I believe that is still somewhere around $15,000.