It _（be) two years since he left school.

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It _（be) two years since he left school.
答案上是is,但是为什么呢?

延续性动词和终止性动词的概念

英语中,动词按其动作发生的方式、动作发生过程的长短,可分为延续性动词和终止性动词.

延续性动词表示能够延续的动作,这种动作可以延续下去或产生持久影响.如：learn, work, stand, lie, know, walk, keep, have, wait, watch, sing, read, sleep, live, stay等.

终止性动词也称非延续性动词、瞬间动词或短暂性动词,表示不能延续的动作,这种动作发生后立即结束.如open, close, finish, begin, come, go, arrive, reach, get to, leave, move, borrow,buy等.
left的动作是很快的,不是长时间维持的动作,不是延续性动词,所以不能用has been
(ps:.终止性动词表示的动作极其短暂,不能持续.因此,不可与表示一段时间的状语连用(只限肯定式).如：

(1)他死了三年了.

误：He has died for three years.

正：He has been dead for three years.

正：He died three years ago.

正：It is three years since he died.

正：Three years has passed since he died.

(2)他来这儿五天了.

误：He has come here for five days.

正：He has been here for five days.

正：He came here five days ago.

正：It is five days since he came here.

正：Five days has passed since he came here.

(1)、(2)句中的die、come为终止性动词,不能与表示"段时间"的状语连用.那么,应如何正确表达呢?可以采用下面的四种方法：

(1)将句中终止性动词转换为相应的延续性动词,如上面两例中的第一种正确表达方式.下面列举几例：leave→be away, borrow→keep, buy→have, begin/start→be on, die→be dead, move to→live in, finish→be over, join→be in/be a member of, open sth.→keep sth. open, fall ill→be ill, get up→be up, catch a cold→have a cold. )

It _（be) two years since he left school. It's two years since he left his hometown.since 为什么不能换成after It __ten years since he left Shanghai. It`s two years since he left Shenzhen 改写同意句 He said to me,"It is two years since I left shanghai."直接引语转化 it has been three years since i left school（be away for）用括号内初二英语、练习选择It is ten years since Jim left school .ow he has __ It ___ 6 years since we left middle school. It -------- five years since he left his motherland. It _____ ten years since he left the army. It _____ ten years since he left the army . 1.It _____ten years since he left the army .