One aspect of the knee that comes in mind immediately may be the patella or the kneecap for the artist. What may not be intuitive is the patella moves with the movement of the knee. With flexion and extension of the knee, the patella moves also.
Note where the patella is at these different poses of the knee. Another thing to keep in mind is the anterior and posterior of the patella. The anterior of the patella is rough because this is where the tendon attaches. The posterior of the patella is smoother in comparison because it needs to slide in the patellar groove.
When you draw the patella, double check that it is where it should be. Do you have the patella too far laterally or medially? Yes this is possible, but this happens when the knee is injured. What is your reference? Are you looking at a hurt knee? At one point I was looking at medical images, DCIM images, to understand the patella. What I had to keep in mind is that most people do medical images of medical patients. Doctors don't usually ask for MRI scans of healthy patients.
At one point in the video while focusing on shading, I had drawn the quadriceps too small. At anterior view, three parts of the quadriceps join together to form the tendon via the patella. So that means you should see it insert into the patella. This is another good thing to check for your drawing. Does it look like the quadriceps are coming together or does it look disjointed?
One last thing to watch out for is the tuberosity of the tibia and the shin. When you shade think about the light source. The tuberosity of the tibia and the shin protrudes anteriorly. That means when you shade you need to think how will the protrusion be shaded differently than the surrounding parts.
Everything is a relationship. Step back and make sure you are not looking at one part. Look at the complete leg and see if it makes sense.