The predicate logic PL is an extension of the language SL.  In SL, the basic units are indivisible simple statements.  In PL, those basic units of SL have an internal structure.  To get a sense of why the internal structures of sentences are important, consider the following argument:

Duff is taller than Ryckman.

Therefore, at least one thing is taller than Ryckman.

This argument is clearly valid, but it is impossible to show why it is in SL. (Why? Oh, please, tell us why?)

In other ways though, PL is quite similar to SL. PL has it’s own derivation system PD (and PD+), but all the derivation rules for SD still apply when deriving arguments in PL. But when working in PL, one often has to use rules of PD (PD+) as well as rules of SD to complete derivations. (AMcD)