Hi dinoram,
Thanks for your question. The flashcards are a great way to procrastinate at work and brush up on your logical reasoning, but I hope that's not all you're doing. The notion of logical opposites is discussed in depth in the Logical Reasoning Bible, which you should be studying as well
Regarding your questions, a "Must Be True, EXCEPT" stem requires you to identify the answer choice that does not need to be true (i.e. not necessarily true, or could be false). The four wrong answer choices will be provably true.
A stem such as "All of the following can be properly inferred from the information above EXCEPT:" is identical in meaning to a "Must Be True, EXCEPT" stem. Functionally, there is no difference between the two.
Onto your second question: the logical opposite of "could be true" is "cannot be true." So, if a stem says, "... each one of the following could be true, EXCEPT", then the four wrong answer choices
could be true, and the one correct answer choice
cannot be. This is why the stem is identified as Cannot Be True.
Once again, make sure to study Logical Oppositions, which is a chapter in the beginning of both the Logical Reasoning and the Logic Games Bibles.
Good luck!