So, from what I understand you are having trouble with every aspect of subdivision so I'll break this response down with each concern you've raised with pictures to go along with it.

Before I do though, it's important to recognize the purpose of subdivision. Lesson 6 (as well as lesson 7) focus solely on the concept of precision. Precision is what you do before you even begin drawing, it's basically planning what you are going to draw. If we take the ghosted lines exercise for example, just by placing the dots of where we are to begin and end the line increases precision. The actual line we draw doesn't influence precision at all because at that point it comes down to accuracy, which is often confused with precision. Here's a picture to help demonstrate the difference. Therefore subdivision is used in both lessons to increase precision so that we can be as precise as possible to increase our understanding of the object in 3D space. They are also used to discriminate tasks so that we can get the most out of the exercise. This is done when they are used with orthographic plans, as by creating and subdividing a plan beforehand allows us to make decisions about where everything will go before we construct the object. This essentially allows us to make 2 / 3 plans of different views of the object and if done correctly, the problem then is figuring out how to transfer the plans to the construction (understanding of 3D space (which is what we want to improve and focus on)). If the plans aren't done then you have to figure out where everything has to go while also thinking in 3D space which not only increases the difficulty of the exercise but at the same time you get less out of it.

So starting with how to determine how much you should subdivide. This entirely depends on you and how far you want to take an object. For example, lets say I wanted to make a plan of this usb. I could do either one of those plans and both would be fine even though one is more comprehensive and precise. It all depends on how far I think I need to go for that specific object. As long as all the important elements have been subdivided precisely so that they can be transfered to the construction then you can subdivide as much or as little as you want.

For subdividing with fractions, there's this imgur album that shows you how to do it. But if you find yourself thinking "Is this 27/50ths across or 29?" when subdividing / planning an object then you should always just round to either 2/5 or 3/5ths because it's not about reproducing the object down to the last millimeter, it's about making as many decisions before drawing the object instead of while we are drawing it. My personal advice is to never go past 10ths as I've never gone further than that and there isn't really a need to. Also you could just stick to the traditional subdivision method of dividing by 2 each time.

Finally I have some tips for subdividing which could help reduce clutter in your drawing and could help with placing elements that don't align to subdivided lines.

• First, only subdivide what is necessary. This may sound obvious and you might think you're already subdividing the minimum amount but there are some tricks you can use which can reduce the amount that you need to subdivide. If you're going for a lot of precision and you have a corner or a small detail that is present elsewhere on the object you can subdivide that small detail first and the shoot the line off across the plan to where the other detail is and build off of that line as shown here (the green line to the right). You can build off of any line this way as long as you subdivided it first because that allows us to replicate it onto the construction. You can also divide quadrants into 3rds (highlighted red in the previous image) and 5ths (top left quadrant of the previous image) and build off of them to precisely place elements.

• Second, this pertains more to the lines themselves. You should always draw as light as you possibly can when drawing subdivision lines and then harder when you draw the lines that shape the object. This will essentially highlight the object in your view and will help you not get lost while drawing (+ it looks better). If you forget to draw light you can always go over the important lines again to try bring the object out of the mess but it's always better to go light.

• Third, make sure you draw curves as straight lines. This isn't really a tip but just a reminder that you should always draw curves as a chain of straight lines before drawing the curve. I mention this because 99% of lesson 6 and 7 submissions I critique always forget this.

I hope this answers your questions about lesson 6! Remember that everything here can also be applied to lesson 7 when you get to it and if anything was unclear or if you have further questions don't hesitate to ask. Also if the pictures were unclear I'm considering doing a live demo of an object and a vehicle to make the process clear so let me know if you prefer that over the pictures.