In the present, the auxiliary verbs must and have to are used to express that doing something is obligatory (You must go), prohibited (You mustn’t go), or not necessary (You don’t have to go).
In the past, however, those distinctions are lost, because must, unlike other modal verbs (can/could, may/might), has no past tense form!
So how do you talk about things that were obligatory? We use had to / didn’t have to or couldn’t / were allowed to:
Present: I must remember to call my hairdresser for an appointment: my hair is huge!
Past: I had to remember to call my hairdresser because my hair was huge!
Present: I have to work late tonight cutting a client’s hair.
Past: I had to work late last night cutting a client’s hair.
Present: You mustn’t tell David our secret, Matt.
Past: You weren’t allowed to tell David our secret, Matt.
Present: You don’t have to tell me his secret, Matt!
Past: You didn’t have to tell me his secret, Matt, but I’m glad you did!
I hope that helps!
Let us know what you think in the comments.