I'm unable to see the question that you have posted this thread on, but from your explanation I gather it is as follows:
Premise 1: not Y -> not C C -> Y
Premise 2: C-most-B B-some-C
Conclusion: B-some-Y Y-some-B
You ask why we cannot also conclude "not B-some-not Y." This is because we cannot negate some statements; all we can do is reverse them. "B-some-Y" is not enough information to say "not B-some-not Y."
Let's consider an example to contextualize this. Pretend we're given the statement "some traditional students have homework." Remember, "some" means at least one, possibly all. We can diagram this as: TS-some-H. We can also reverse it to say that some people with homework are traditional students, or H-some-TS. However, we cannot negate the original some statement to say that "some non-traditional students do not have homework." Perhaps all students have homework, we don't have enough information to say.
Does that make sense? Please let us know if you have any other questions!
