Any given categorical proposition can be presented in 4 different ways. So,
GAR, which is affirmative is the same proposition as GER (complement), which is negative, which is the same as R (complement)EG, which is negative, which is the same as R(complement)AG(complement), which is affirmative.
A given proposition as it presents itself will be affirmative or negative, and its quality is a function of the copula. A and I = affirmative; E and O = negative.
Ordinary language is often ambiguous. ‘Not all men are fat’ is the denial of a universal affirmative: not(All men are fat) and, as you have or will see on the Square of Opposition, the negation of an A-type proposition is an O-type proposition. Where M: men, and F: fat, the proposition is translated as MOF.
‘All men are not fat’is best translated as being equivalent to ‘Not all men are fat’and so translated as MOF. However, sometimes, speakers use this way of saying things to express a universal negative, i.e. to mean ‘No men are fat’ which is translated as MEF.
If you’re speaking to someone or in correspondence with them, then you can ask them to clarify what they intend to claim. If, however, you’re not able to do this, then the Principle of Charity comes into play. This principle says that in cases of ambiguity, you will assume that your interlocutor is make a particular claim rather than a universal claim, universal claims being harded to defend.