If an 18-wheeler jackknifed and caused my accident and injuries, does that mean the driver was negligent?

That depends entirely on the circumstances surrounding the accident. If the driver was forced to brake suddenly or turn abruptly to avoid a collision, or if there were unforeseeable road conditions that resulted in the rig jackknifing, the driver may not have been negligent. However, if there was no valid reason for the driver’s actions that caused the tractor-trailer to jackknife and resulted in your accident, the driver may have been negligent.