Can retainer align my underbite teeth?

Nope: Retainer's retain, they don't correct. If you have an underbite, chances are you have both dental and skeletal (jaw) malrelationships, and removable appliances don't correct jaw disharmonies. Ask your dentist to refer you to a fully qualified orthodontic specialist for exam & initial evaluation. First visit is often complimentary or nominal cost. Ask a pro for advice.

No: You need to see an talented general dentist or orthodontist for a thorough evaluation. Depending upon the extent of your problem, they may recommend orthognathic surgery to correct the jaw size discrepancy. This would be a combination of oral surgery and orthodontic treatment. Often the results are amazing.

Yes, but: Very, very difficult. The retainers would be something like invisalign and would typically be called aligners. I would see an orthodontist to discuss but be prepared to hear that your underbite is too difficult to address with aligners. These are tough ortho cases. We would likely not treat with aligners.

Possible correction: If the jaws align correctly and the lower teeth have spaces between them to allow retraction towards the tongue, alignment may be possible. This can be a complicated evaluation so talk to an orthodontist.

No.: Retainers retain, they do not move teeth. To correct an underbite you will need braces or invisalign, and sometimes surgery.