Let me try: "Understand" is an an infinite verb. "Understood" and "did" are finite ones. Each clause can have only one finite verb (its head) so "did" and "und

Let me try: "Understand" is an an infinite verb. "Understood" and "did" are finite ones. Each clause can have only one finite verb (its head) so "did" and "understood" cannot both occur in one. Put another way, "did" only takes an infinite verb as its argument.