My knee hurts for about a year what could it be?

Get Xray: It is most likely arthritis. You should go to doc to have an xray done. There are other causes such as meniscal and acl/pcl damage which shows up on an mri.

Multiple reasons: Since the pain is going on for a year it does not sound like a bruise or a strain since those symptoms usually resolve in days to weeks. Problems in a joint for a year do suggest the possibility of an arthritic problem and should be evaluated particularly if there is inflammation. A structural problem such as a meniscal tear could cause prolonged symptoms in a knee and again should be looked at.