Hopefully this helps. In #2, it says "some courses ARE open" while #4 is "some alphabets ARE NOT phonetic." It's because one statement is written in the positive (so it negates to negative) while the other is written in the negative (so it negates to positive).

So: if some courses are open to undergrads, the negation would be that no courses are open to undergrads. That is the only way to make it false that undergrads can take some courses - if they can take none. If it had been written like "some courses are NOT open to undergrad" then the negation would have been "ALL courses ARE open to undergrads."

I think in this case, the word "solely" helps the argument. We're trying to connect not accepting the coupons, to the restaurant doing it to hurt the competition. "Solely" in this case can read like "only." So the ONLY reason a restaurant won't take a coupon is to hurt the competition. By saying solely, it removes the possibility that they didn't accept the coupon for a different reason. So if the only reason they won't take a coupon is to hurt the competition, and they did not take the coupon, the must have wanted to hurt the competition.