As we have already begun to do, we will use letters to represent (possibly variable) logical statements and substatements. To complete the conversion from verbose language to compact symbolism, we will introduce symbols to represent the Five basic connectives.
Using variables to represent statements and the above symbols to represent connectives allows us to isolate the task of analyzing logical structure, without being distracted or influenced by the content of the statements.
Using substatement variables is not the same as using free variables. You should think of substatement variables as placeholders for specific logical statements which, by definition, can each be determined to be either true or false.