Holing CTRL, and left-clicking on a sentence will select a sentence as a reference sentence. The current line's reference sentences are highlighted in violet. Only a line before the current line can be selected as a reference. In addition, if the sentence is in a different subproof than the current line, then the sentence can not be selected as a reference. Each rule requires a specific amount of references (see Rules Index).
Holding SHIFT and left-clicking on a sentence will select the sentence. The sentence will be highlighted in red-orange. Multiple sentences can be selected this way, however when another action is taken, all of them will be de-selected. Pressing CTRL+K will kill (cut) the selected lines, and CTRL+G will copy the selected lines. If no lines are selected, then the current line will be used.