I can't keep my leg straight without pain. what could cause this?

Check knee caps: The most common and likely cause of pain in front of the knee is an irritated or early wear of the knee cap cartilage or the surface cartilage of the thigh bone that it rubs. Other causes could include a plica, patellar tendonitis etc. For starters avoid stairs, knee extension machines, deep knee squats, lunges etc. Try therapy, antiinflammatories, ice and sometimes a knee sleeve. See orthopedist.