Thank you so much Yu Chen :)

Here's a really useful rule I learned: All parallel lines converge to a common vanishing point. If you have something like an octagon with 4 sets of parallel lines, there will be 4 different vanishing points etc. With a cube, you have 3 sets of parallel lines with each set containing 4 lines, so you have 3 vanishing points with 4 lines converging on each.

Great work, these are looking really expressive! I'd say that perhaps you could get a little more form in the limbs. i feel like they may be getting a bit lost in some drawings. i also feel like there's a bit of a disconnect between the upper and lower body. i think it may be because you are stopping the line of action half way down the body and then picking it back up again in the legs. i think this can have the effect of breaking the gesture. maybe try and get a more continuous line of action defining your idea for the pose through the body. and use the same idea for the limbs, look at how they flow from one to the other.

Pretty good,You definetly nailed the proportion and volume.I suggest now trying to shade the face and understanding the strcuture of the feature so you can describe the form correctly. (These two series are really good for that https://youtube.com/playlist?list=PLflflDShjUKH4EfZyf0vuKEuqeqvlV0Qd https://youtube.com/playlist?list=PLR2KBLDDnZz0pHBiiyrqlOB3FU-W5XX1k)

Wow, this looks really clean! I'm not too experienced with figure drawing so take this with a grain of salt but in the last figure drawing, it seems like the flow is really disjointed between the torso and legs. It was kind of awkward? I don't know what the original pose looked like so I can't give any detailed impressions

My impression is that it is not the shoulder piece that is “wrong”, I think you are on the right track considering the alignment of the rest of the body as being necessary for it to read. I think also perhaps you might be expecting too much clarification from this stage of rendering. If the overlap on the left hip was slightly more defined it would help show the lean of the torso on the forward/backwards axis. The main issue though is as I think you pointed out, the undefined ribcage. On her right side you’ve drawn what is essentially a straight the whole way through to just try and nail that gesture, but you’ll notice a slight dip inwards where her ribcage is defined under the volleyball. That detail could be the definition you need to orient the ribcage as slightly angled and with the previous note of the overlap of the hips/torso might give enough definition for the viewer to see the relationship between them. So to sum up, I think there are subtle but important details that got lost in your line work (specifically on the overlap on the left where you repeated strokes a few times ) and the angle of the right side isn’t quite right which doesn’t completely fill in the information you are trying to define in relation to the shoulder. All of that said, I think you did a great job and certainly are on the right track, keep at it!

My guess is after some time, you should have a fleshed out central theme, any things that take away from it are considered mistakes, while others are considered flairs. I like to use spikes as an example. They're good on a leather jacket of a motorcycle gang member, but awful on a nurse(if you're trying to portray the generic personality traits associated with nurses, instead of something ironic).