Can the plastic tray be sanded down in way it is not sticking out? It's cutting like a knife. Under my tongue is hurting bad.

The first aligner should be fitting pretty well. It sounds like your aligners are ill fitting and may need to be remade. Contact your orthodontist to have it evaluated. If filing it down still does not help, and your orthodontist cannot improve the fit with changing the attachments, new impressions will need to be taken. Good luck!

Thank you for bringing your question to RealSelf! While some discomfort is to be expected when beginning Invisalign, it shouldn't be characterized as pain. Minor adjustments can be made to Invisalign trays. You should call your dentist right away to have this done, so you don't become discouraged from wearing your aligners and getting the best possible results.

This should be an easy fix where the dentist will sand down the sharp edge to make it more comfortable. In the meantime you could not wear the tray or place some wax on the sharp edge to help with the pain.