They understood not (ou sunhkan). First aorist active indicative (one of the k aorists). Even Mary with all her previous preparation and brooding was not equal to the dawning of the Messianic consciousness in her boy. "My Father is God," Jesus had virtually said, "and I must be in His house." Bruce observes that a new era has come when Jesus calls God "Father," not Despote. "Even we do not yet fully understand" (Bruce) what Jesus the boy here said.