What is the past participle of must?

The verb "must" is a modal auxiliary verb. Modal auxiliary verbs do not have past participles.

Instead of using the past participle of "must," you would use a different verb to express the idea of necessity or obligation in the past.

Here are some examples:

* "I must go to the store." (present tense)

* "I had to go to the store." (past tense)

* "I was obliged to go to the store." (past tense, using a different verb)

* "I was required to go to the store." (past tense, using a different verb)