Adam Clarke on John 13:12

Know ye what I have done - Our Lord had told Peter, in the presence of the rest, Joh 13:7, that he should afterwards know what was the intent and meaning of this washing; and now he begins to fulfill his promise; therefore I think it more likely that he gives a command, here, than asks a question, as he knew himself that they did not comprehend his design. On this account γινωσκετε might be translated in the imperative mood, Consider what I have done.