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.

Related Questions

Is my sore knee serious? I am 65 years old, 6'2" tall, and 190 lbs. I run about 5 miles twice per week. I run three or four 10k races a year, times between 48 and 49 minutes. Recently my right knee hurts and loses power when going up stairs.

Impossible to say. Have the knee evaluated by a rheumatologist or orthopedist and then you will know.
Knee pain. Anterior knee pain from chondromalacia is common, as are degenerative medical tears that may catch in certain positions loss of power could be secondary to pain, however if there is pain that is unusual for your own regular regimen best to get evaluated, get some X-rays and either find out if there is anything up and if not. .. get piece of mind.

I had meniscus surgery about 3 years ago they removed dead cartilage now my knee hurts if I stand to long and gets worse at night what could it be?

Couple things. It could be arthritic pain it could be patellafemoral pain syndrome. Depends where pain is. U need to do quad and vmo exercises. See if improving function of knee improves condition. If not cortisone or visco inj may help. Talk to ur local orthopedic/sports med doc.

Left knee hurts, clicks and grinds. Happening for years, stairs and chairs are challenge, and road trips. Only 19, confused about issue and what to do?

Further work up. You deserve further work up. If you've not had a doc look at your knee, that's where you need to start. A through history and physical needs to be the first step. Xray or MRI may be considered based on the initial assessment.
Need orthopedic eval. Needs orthopedic evaluation. Could be simple instabiltity, but more likely an intrinsic knee problem.