I was diagnosed with a bone tumor in my frontal/ethmoid sinuses 1year ago so far ive had 4 operations.the reason we had to do the operations was that it was pushing my eye.what can actually happen if the brain will be left bare? what choice do i have?

No, it cannot be left unprotected. Neurosurgeon and plastic surgeon will take care the opening with autograft.

The neurosurgeon can place a patch graft if the dura mater must be excised along with the bone, to prevent a cerebrospinal fluid leak. Compressive optic neuropathy is when a tumor presses on the optic nerve and it can cause permanent blindness. Coordinate the neurosurgical care with neuro-ophthalmology to monitor the peripheral visual field and visual acuity.

the brain isn't "left bare" part of the surgery is a reconstruction as well as the resection, removal and then recon with multiple varied things possible, but usually your own bone is most common (or bone grafts from another area) so as to reconstruction sinuses in front of adjacent to area