1.祈使句首
如:Be careful,there're bears hidden in the bushes.小心点,灌木丛中有熊出没.Don`t be late!不要迟到
2.前面跟情态动词的时候
如:He will/should/must be here then.那时他将/应该/必须在那儿.
3.连接不定式
如:What do you hope to be in future?你将来希望从事什么工作?