What if i missed (not mailing the forms) the 15th April 2019 deadline for 2018 federal tax return?

[rjs]

Mail it now. If you are getting a refund there is no penalty for filing late. If you owe tax, pay what you owe. After you file, the IRS will send you a bill for interest and penalties. The sooner you file, the less the interest and penalties will be.

[Critter]

Mail in the return NOW.   If you have a refund there are no consequences.   If you have a balance due send the payment with the return and the IRS will bill you for any penalties and interest you owe.