it is not true that all S are P
rephrased: Some S are not P
Some S are not P
Some S are P
Wrong:
this says for at least one x if x is S then x is P.
True: If a person has 7 heads then that person is peculiar
False: Some seven headed people are peculiar
(Since there are no seven headed people)
. . . It is not the case that some S are P
. . . rephrased: No S are P