i've had knee problem for many years,every now and then i get stretched ligaments, which dont allow me to bent my knee all the way,for a few days till it heals. is it ok to still train my legs, do excercise that i can do, like leg extensions and leg curls? i cant do squatts and leg presses.

Too much sheer force on the tendon. Due to the original injury my knee cap wasn't tracking straight up and down correctly and it's still a bit out of whack. I'm OK to do all kind of presses and squats if I wear knee wraps to keep the knee cap in place. Like tjwes said, SEE A DOCTOR!!! My main problem came from putting it off too long and having tissue deterioration. You need to get it fixed. Also, form is of utmost importance. Allways, always, ALWAYS press with your heels. That takes the stress off your knees and puts it on your muscles.