Great post! Thanks!
I have a quibble with example 2, though, which reads:
All men are idiots
Tom is an idiot
Therefore Tom is a man; this conclusion is perfectly fine and acceptable in GMAT. But, when we see carefully this same stimulus can be logically valid without being factually correct
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B: Set of all idiots
A: Set of all men (contained in B)
C: Tom (can fall outside A) . . . so, you see that TOM could be a man or could not be a man; irrespective for purpose of CR in GMAT, the conclusion 'Tom is a man', is perfectly acceptable
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It seems to me that the premise that all men are idiots does not imply that no women are idiots, thus the argument would be fallacious as written. Tom may still be a man or a woman. To correct, the order of the first two nouns in the first premise should be switched.
All idiots are men.
Tom is a idiot.
Therefore Tom is a man.