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by 和 till 均可用作介詞，表示時(shí)間，意思是“到……時(shí)候”。具體區(qū)別如下：
1. by 的真正意思是“在……以前”、“到……時(shí)已經(jīng)”、“最遲到……”等。如：
By now he will be eating dinner. 現(xiàn)在他一定是在吃飯。
I'll have finished this book by tomorrow. 到明天我就會(huì)把這書看完了。
I require my new coat by Monday. 我的新上衣要在星期一以前做好。
I'll finish the work by Saturday. 我將在星期六以前完成工作。
You must get the work done by Friday. 你必須最遲在星期五以前把工作做完。
You never know, it may stop raining by tomorrow morning. 說(shuō)不定這雨到明天早上就不下了呢。
The outstanding debts must be paid by the end of the month. 未了的債務(wù)須在月底前償還。
2. till 的意思是“到……為止”，其謂語(yǔ)動(dòng)詞應(yīng)是持續(xù)性動(dòng)詞，若為終止性動(dòng)詞，則應(yīng)為否定式。如：
He waited till Friday. 他一直等到星期五。
He didn't arrive till after midnight. 半夜過(guò)后他才到。
I won't qualify till next year. 我明年才具備資格。
I needn't leave till 9 tomorrow. 我不必在明天9點(diǎn)以前離開。
I needn't be leaving till 9 tomorrow. 我不必在明天9點(diǎn)以前離開。
We shan't know the results till next week. 我們下星期才能知道結(jié)果。