Why i can't stretch my hamstring without feeling pain in the calf? i have been practicing yoga for 3 years but i don't feel any calf improvement.

See below: Go see a physical therapist who specializes in the back and lower extremities and i would also recommend seeing an orthopedic surgeon for just some basic x-rays , evaluation and see if you have any underlying structural issues which would lead to your mechanical problems.

Keep trying: When you stretch the hamstring, you also stretch the other muscles in the back of the leg. It is very common for people to have tight achilles and calf muscles, so keep going. When you are in forward bend in yoga, bend your knees and then gradually straighten them, using your breath to deepen the stretch. Stretching other areas of the leg and even gentle spinal twists can also help release.