The doctor says I have a torn ligament and he wants to do surgery on my left knee to improve it does it get better after surgery?

It should improve. Certain ligaments need to be repaired, your dr makes the call based on the exam and history. The knee uses the acl as its primary stabilizer with remaining muscles, ligaments and menisci helping. If the acl is not repaired in active people it leads to early onset arthritis. Be prepared to rehab your knee for several months.