Some familiarity with either system or with natural deduction calculi will be required when using the Proof Builder. If you feel not sufficiently familiar with any of these systems, you might consider having a look at the other functions of the Gateway to Logic .
Propositional expressions consist of propositional variables, connectives and brackets.
Note that most tasks accept propositions which start with a lowercase letter, too. Since the proof checker does not, you should probably always use capitalized propositional variables. Although this might seem strange at the first glance, especially since in the case of the other connectives there is no such requirement, there is a simple reason. Although this is not wrong, it is sometimes misleading. Although a strict formal language usually requires brackets, the gateway allows you to leave them out.