My dads left knee has swollen up and is sore when he moves it. what could be wrong with it and what should we do?

Arthritis.: A sore, swollen knee, most likely with fluid in it, is caused by arthritis (inflammation in the joint). Cause usually is osteoarthritis in an older adult, but there are many ohter causes such as injury, gout, and more serious types of arthritis. He should see his doctor if it persists, especially if anti-inflammatory medications like Ibuprofen don't help.