There is no past or future tense form of this verb. Must is a modal verb and shows a command or something urgent. If you want to express this in the past or future you will have to use some other way, like a temporal tag or using it with the perfect tense. For example: --------------------------------------------------------------------------- Yesterday I told her she must not go outside. (temporal tag) Two weeks ago I must have forgotten to sign the check. (used with perfect tense) When I was young, my mom must have taught me this. (Used with perfect tense). --------------------------------------------------------------------------- It is mostly the same with the future tense, but we would not use future perfect. I will tell her she must not go outside. Tomorrow I must go to the store. (Temporal tag) Next year I must go back to college. (Temporal tag)