Some movie elephants are not beings which are flightless. Many movies are not intellectually challenging forms of entertainment. Notice that the four definitions refer to the entire statement. It can only fit one of those four categories and it must fit one. If a statement begins with the term itself and not with all or some then we make the assumption that it refers to all.
Crows are birds has the same meaning as All crows are birds.
The term itself we call distributed or undistributed. It is distributed when it refers to the entire class and undistributed when it only refers to part of the class. As we might expect, each of the four classes of statement have a different pattern. All crows are birds. Some birds are flightless birds.