be也是动词的一种,叫做状态系动词,是状态系动词is和are 的原形.情态动词后应该跟动词原形,本来正常时态（一般现在时）的句子应该是There is a bridge over the river.用should是想要表达应该有的意思,所以在原句基础上加上should,变为There should is a bridge over the river,但是因为情态动词后面要跟动词原形,所以就要把is变成它的原形be.第二句也是同理,本来原句是it is much better with some milk in it,但是是要表达将来的意思,翻译过来就是,加些牛奶它（咖啡）将会更好一些,所以就变成it will is much better with some milk in it.同理will后面要跟动词原形,所以要把is变成原形be.