Cherries are red, but some apples are not. So cherries aren't apples. Some apples are green so, since all green apples are sour, some apples are sour. Many motorcycles are loud. Harleys are loud, so Harleys must be motorcycles.
Some long books are hard to understand.
War and Peace is a long book, so it must be hard to understand. Some alcoholics should not become cops because they are irresponsible, and irresponsible people shouldn't be cops. Many college student work to earn money, but some students don't work. Those who don't, don't earn any money. If Sam goes to the movies, she can't go to the party.