Until用法詳解表示“直到……為止”,兩者常可換用,只是 until 比 till稍正式,所以在正式文體中,一般用 until,而在口語或非正式文體中則兩者都可用.從其后所接成分來看,till多與名詞或較短的從句連用,而較長(zhǎng)較復(fù)雜的成分多用 until,位于句首時(shí)也多用 until.
兩者在用法上應(yīng)注意以下幾點(diǎn)：
1.相關(guān)主句謂語必須是持續(xù)性動(dòng)詞,若是終止性動(dòng)詞,則應(yīng)為否定式（因?yàn)榻K上性動(dòng)詞一旦被否定就成為狀態(tài),便可持續(xù)）.如：He waited until she was about to leave.他等著一直到她準(zhǔn)備離開.She didn’t marry until she was over fifty.她直到50多歲才結(jié)婚.
2.引導(dǎo)時(shí)間狀語從句時(shí),其謂語要用現(xiàn)在時(shí)表示將來意義.如：I won’t leave until he comes back tomorrow.我要等他明天回來后再走.Continue in this direction until you see a sign.一直朝著這個(gè)方向走就看見指示牌了.
3.有時(shí)其后可跟副詞或介詞短語等.如：He has been ill until recently.他最近一直生病.Until when are you going to stay here?你在這兒要待到什么時(shí)候?We didn’t eat till past midnight.直到午夜過后我們才吃飯.She won’t go home until after the exam.她要考完試之后再回家.如果語境的需要,有時(shí)甚至可以接帶有“引導(dǎo)詞”的從句.如：Don’t leave until after I’ve spoken to you.我和你說完后再離開.
4.用于 not…until [till]…結(jié)構(gòu)
(1) 若主句動(dòng)詞為終止性動(dòng)詞,其意為“直到……才……”；若主句為持續(xù)性動(dòng)詞,其意為“不到……就……”.比較：They didn’t find her till the next day.他們直到第二天才找到她.He didn’t wait until I returned.他沒等到我回來（沒等我回來他就走了）.有時(shí) until 或 till 可以換成 before.如：Until [Before] you told me,I had no idea of it.在你告訴我之前,我對(duì)此一無所知.（即直到你告訴我,我才知道）
(2) 可用于It is (was) not until [till]…that…這樣的強(qiáng)調(diào)句型.如：It was not until I met you that I knew real happiness.直到我遇到你以后,我才真正體會(huì)到幸福.It was not until she took off her dark glasses that I realized she was a famous film star.直到她取下墨鏡,我才意識(shí)到她是一位著名影星.
(3) 若將not until [till]置于句首,其后的主句要用倒裝語序.如：Not until much later did she learn who her real father was.她很久以后才知道她真正的父親是誰.Not until he received her letter did he fully understand the depth of her feelings.他直到收到她的信之后才充分理解她的感情有多深.