"He must have" or "He must has"?
Have...... grammatically correct
I understand where you're coming from. 'If he hasn't undone it, so technically he still has done it up to this point, it must be 'he must has''. But no, that would still be 'he must have'—'have' is still present tense.
Have would be correct.
"He must have" is the grammatically correct option (: