The modal verb MUST is irregular. We actually use "had to" for the past form so you should say "I had to study". I hope that this helps!

I also would use MUST HAVE+Past participle of the verb for another reason too: - Exaggeration or to imply thought "hmmmm i must have studied for at LEAST four hours last night" and - When I believe something to be true "I must have given the right answer!" You would not say "I must have studied yesterday" instead of "I had to study yesterday" although both sentences are grammatically correct. The first sentence would be understood to mean that you cannot remember whether you studied or not. The second sentence would be understood to mean that it was vital for you to study - it was a necessity.

"Must" itself has not past form and we use "had to". Use "must" only in present tense.

"had to" I must study now. I had to study yesterday.

MUSTED