Jesus answered and said unto him, What I do thou knowest not now; but thou shalt know hereafter. What I do thou knowest not now, etc. - As if our Lord had said, Permit me to do it now, and I will shortly explain to you the nature of this action, and my motives for doing it.
Thou shalt know hereafter - Μετα ταυτα, after this business is finished. And so we find he explained the whole to them, as soon as he had finished the washing: see John 13:12-17. I cannot think that this refers to any particular instruction received on this head after the day of pentecost, as some have conjectured.