Ho dovuto studiare = I had to study
Note that the verb 'must' can't be conjugated. In the sense of need/necessity/obligation to do something, 'must' only exists in the present tense. For other tenses, we use the synonym 'have to' - for example:
Dovrò studiare = I will have to study
The form 'I must have studied' exists, but it has a different meaning. It's a supposition about the past, equivalent to 'Devo aver studiato'.
Once again, I hope my Italian was OK and I hope that this has helped.