be也是動(dòng)詞的一種,叫做狀態(tài)系動(dòng)詞,是狀態(tài)系動(dòng)詞is和are 的原形.情態(tài)動(dòng)詞后應(yīng)該跟動(dòng)詞原形,本來(lái)正常時(shí)態(tài)（一般現(xiàn)在時(shí)）的句子應(yīng)該是There is a bridge over the river.用should是想要表達(dá)應(yīng)該有的意思,所以在原句基礎(chǔ)上加上should,變?yōu)門(mén)here should is a bridge over the river,但是因?yàn)榍閼B(tài)動(dòng)詞后面要跟動(dòng)詞原形,所以就要把is變成它的原形be.第二句也是同理,本來(lái)原句是it is much better with some milk in it,但是是要表達(dá)將來(lái)的意思,翻譯過(guò)來(lái)就是,加些牛奶它（咖啡）將會(huì)更好一些,所以就變成it will is much better with some milk in it.同理will后面要跟動(dòng)詞原形,所以要把is變成原形be.