I was reading these paper(dont really remember the title) it stated that there are simple arguments that are clearly valid but would be counted as invalid in the sentence logic system it was using. i don't understand how this can be.. can anyone give me an example that satisfies the statement made. this is an example i saw from the paper.. "every class is easy is valid in predicate logic but not in sentence logic because you can deduce from it that philosophy is easy." i still don't understand i.e can't form an example of my own.
The example you give (at least in the way you formulated it) doesn't make sense to me, because "Every class is easy" is obviously not valid in PL and neither does it allow for the deduction that "Philosophy is easy" without further axioms.
What I imagine could be meant is that sentential logic is not powerful enough to formalize the linguistic details that are needed to derive certain validities.
Take the standard textbook example
All humans are mortal.
Sokrates is a human.
∴ Sokrates is mortal.
Intuitively, this should be a valid argument, and in predicate logic, it can be formalized as
∀x(Human(x) -> Mortal(x))
and proven proven to be valid using the rules of universal instantiation + modus ponens.
But in sentential logic, where there are no predicates and quantifiers, the most fine-grained formalization we can get (since there are no sentential connectives involved in any of the sentences) is
which is obviously invalid because p ↦ True, q ↦ True, r ↦ False is a countermodel.