Hitting Ctrl+J adds a conclusion to the proof. A conlusion is always added after the current line, or, if the line is a premise, then the conclusion is added after the last premise. If the current line is a conclusion, then it is highlighted in cyan.
Hitting Ctrl+P adds a premise to the proof. A premise will always be added after the last premise.
Pressing Ctrl+B adds a subproof to the proof. When in a subproof, a conclusion will always be added within the subproof. The first line of a subproof does not require a rule, but instead acts as a premise. Pressing Ctrl+D ends the current subproof. This creates a new conclusion just after the subproof.