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英語六級語法知識：since的四種用法
1) since +過去一個時(shí)間點(diǎn) (如具體的年、月、日期、鐘點(diǎn)、1980, last month, half past six)。
I have been here since 1989.
2) since +一段時(shí)間+ ago
I have been here since five months ago.
3) since +從句
Great changes have taken place since you left.
Considerable time has elapsed since we have been here.
4) It is +一段時(shí)間+ since從句
It is two years since I became a postgraduate student.
英語六級語法知識：一般現(xiàn)在時(shí)代替完成時(shí)
1) 表示即將發(fā)生的或預(yù)定中計(jì)劃好的活動。
Are you staying with us this weekend? 這周和我們一起度周末嗎?
We are leaving soon. 我們馬上就走。
2) 漸變動詞，如：get, run, grow, become, begin及die。
He is dying.
英語六級語法知識：延續(xù)動詞與瞬間動詞
1) 用于完成時(shí)的區(qū)別
延續(xù)動詞表示經(jīng)驗(yàn)、經(jīng)歷; 瞬間動詞表示行為的結(jié) 果，不能與表示段的時(shí)間狀語連用。
He has completed the work. 他已完成了那項(xiàng)工作。 (表結(jié)果)
I've known him since then. 我從那時(shí)起就認(rèn)識他了。(表經(jīng)歷)
2) 用于till / until從句的差異
延續(xù)動詞用于肯定句，表示"做……直到……" 瞬間動詞用于否定句，表示"到……，才……"
He didn't come back until ten o'clock.
他到10 點(diǎn)才回來。
He slept until ten o'clock.
他一直睡到10點(diǎn)。
典型例題
1. You don't need to describe her. I ___ her several times.
A. had met B. have met C. met D. meet
答案B. 首先本題后句強(qiáng)調(diào)對現(xiàn)在的影響，我知道她的模樣，你不用描述。再次，several times告知為反復(fù)發(fā)生的動作，因此用現(xiàn)在完成時(shí)。
2.---I'm sorry to keep you waiting.
---Oh, not at all. I ___ here only a few minutes.
A. have been B. had been C. was D. will be
答案A. 等待的動作由過去開始，持續(xù)到現(xiàn)在，應(yīng)用現(xiàn)在完成時(shí)。