I have an underbite but my molar and pre molar teeth are aligned. i have spaces on my lower front teeth. do i need jaw surgery to fix it?

Can't tell: I would have to examine you to answer that question. From your description, however, it's possible that your problem may be resolvable with Orthodontic treatment alone. See a qualified Orthodontic Specialist for evaluation.

Orthodontist eval: Typically, underbites and overbites can be corrected by any number of means such as braces and corrective gear. Surgery is an option of there is misalignment or maldevelopment needing to balance the skull and jaw areas. A good thorough Orthodontist evaluation should be able to tell you if you will need surgery or not.

Orthognathic surgery: Jaw surgery is more likely required when there is a skeletal or jaw discrepancy present rather than a tooth alignment problem. Most cases can be treated with conventional Orthodontics. You should see at least one Board Certified Orthodontist for a proper workup and evaluation and consultation in order to get accurate answers to your questions that cannot be given over the internet, sight unseen.