RissRoss: ER Questions asks some rather good detailed questions. I don't have the coursepack in front of me, but I think I can provide some "philosophical" guidance that might help.
1) In the coursepack, when talking about subtypes, it mentions "optional subtyping is represented by leaving a single line from the supertype to the circle." My question is, what is "circle" referring to? 2) Are subtypes necessary or these entites always abe to be represented as regular entity types with additional relationships attached to them? 3) Referring to the ER end of chapter questions in the coursepack, could the answer to question 3, part 2 be drawn up using a ternary relationship to express the relatoinship between employee, skill and machine type? It is not shown this way in the chapter answers, but I thought it was a possibility.
In regards to Question 1, I'm not sure, but if you refer to the in-class example (ER2), you see that the subtype is represented by a single line. BTW, this is an area that I said would not be heavily represented on the exam.
I'm not sure I entirely get Question 2, but my main observation is that subtypes are more of a finessing move (i.e., you can make it by without them with only a small loss in your ability to model a situation).
Question 3 is interesting. I think a ternary relationship could be used. The key feature of a ternary relationship is that you cannot enforce binary constraints between entities. Mind you, one does not rule out the other in most instances. You can have both in many cases.