The subject of both statements is the same. The subject of each is a rectangle. An indefinite subject is always a subject just as a definite subject is always a subject.
The following conditional statement (1) is true.
(1) If a rectangle is a square, then some rectangles are squares.
However, its inverse (2) is false.
(2) If a rectangle is not a square, then it is not true that some rectangles are squares.
Since the contrapositive of a conditional statement is logically equivalent to the conditional statement, the contrapositive of (2), (3), is also false.
(3) If some rectangles are squares, then a rectangle is a square.
However, (3) seems to be an implicit rule of inference that is used in my argument, as I quote.
Since (3) is false, it has a counterexample. The counterexample is: some rectangles are squares, but a rectangle is not a square.
So it appears there’s a fallacy in my argument where I’ve quoted it. However, the argument I’ve previously cited at twitter.com/paulemok/status/975234801409118208 avoids this fallacy. It does not have “some rectangles are squares” as a premise.
They are likewise contradictory statements. Those statements have the same subject, something. What that something is does not matter. Whether that something is different in the second statement from what it is in the first statement does not matter. As long as we are considering something, we are not violating any rules.
As I’ve previously explained, it is commonly used and accepted. It is not just personally used and accepted by me; it is personally used and accepted by society in general. It’s not just used and accepted in the United States, either. I have a book apparently from Canada, which claims to be published outside of North America as well, that invokes such language use. Its name is Modal Logics and Philosophy, 2nd Edition (2000, 2009) by Rod Girle. The book claims to be printed and bound in the United Kingdom.