什么时候be动词用原型be？

不定式+动词原形 ,所以be要用原形
情态动词+动词原形,所以be要用原形,即be
凡是后面要用原形的都是用be,而不是am is are was were have/has/had been

He should be a teacher.
情态动词后根动词原形。
He must be very tired。