Therefore Socrates is mortal. Socrates is a man. Therefore Socrates is green. Socrates is a man. Therefore Socrates is green.
Some men are mortal.
Socrates is a man. Therefore Socrates is mortal. In example 2, the first statement is false, but the form or structure of the argument is correct or valid. In examples 3 and 4, the first statement says something about some men, not about all men. One could correctly reason from this first statement that Socrates might possibly be green or mortal, but he could not correctly reason that Socrates necessarily is green or mortal.