Must doesn't have a past form. Instead we use 'HAD TO + VERB': I HAD TO GO to the bank. I HAD TO GET up early. We cannot say: I musted went to the bank / I must went to the bank, or, I musted got up early / I must got up early. Because HAD TO is indicating the past, the main verb is in the present tense. It also means that we had an obligation to do something. We had no choice but to do it. MUST is used with present perfect (MUST + HAVE + PAST PARTICIPLE). I MUST HAVE LEFT my keys in the car. (I cant find them anywhere else) HE MUST HAVE GOT (AmE. gotten) in through the window. (All the doors were locked) This means that there was a very high chance that something happened. I hope this helps. All the best, Lee.

Past Perfect of "must" is "had had to" and is rare. Here's one: If I hadn't had to go there, I wouldn't have gone.

There is not a simple past tense of "must". You need to use "had to" The past perfect of "must" has a different meaning than "had to", for example "The lights are on, they must have arrived home" means the person makes a prediction. Examples: I had to understand. You must've understood. Apparently, he had to quit his job to be free. To be able to travel that much, he must've quit his job. All he had to do was to press the button. Someone must have pressed the button!