Yogis … please always be very aware of your knees in your standing poses, especially Warrior poses.
Make sure that the knee of your front bent leg does not move out in front of your ankle. Allowing your knee to move in front of your ankle can be very dangerous for your knee, especially if you do it like this pose after pose, day after day. If you already have weakness in your knees, this can definitely exacerbate it.
Like Baz Luhrmann said in his song Everybody’s Free (To Wear Sunscreen), "Be kind to your knees, you’ll miss them when they’re gone."
Your shin should essentially be perpendicular to the ground. Maybe even think of it as stacking your knee over your heel. From your visual perspective, this might allow for better alignment. If you need more space in your stance to allow this to happen, simply slide your back foot back a bit.
So often in classes I see students wanting to go deeper into their standing postures, which results in lunging forward, bringing the knee too far in front of the ankle. This places too much strain on all parts of the knee. In order to go deeper, consider lowering down from the outer hips while keeping the front knee stacked over the ankle. As you lower the outer hips, continue to feel a lift in the pelvic floor. Keeping the back leg really strong and committed will help to stabilize and deepen the pose as well. Maintain a lot of strength in the back leg by lifting the inner ankle to the outer ankle and the inner knee to the outer knee while pressing down through the outer edge of the back foot.
You really shouldn't ever feel any pain in your knees. If you do, please back off and back up out of the pose.
Be good to your knees. Be good to yourself. Take care of your knees. Take care of yourself.