before的用法永久说不够
一、It be some time +before…
此句型紧张有四种涵义，分散论述如下：
1.该句中主句为将来时态，before从句为寻常如今时态；主句为寻常已往时，从句通常也为已往的时态。
2.比如：
It will be some months before we graduate from school.(过几个月，我们就毕业了。)
Shall we see him again? (我们还能见到他吗？)
I think so. It won’t be long before he comes back to the hometown.(我想会的，过不了多久他又前往故乡。)
该句型中的时间长为long, a long time, years, months, weeks, days, hours, minutes等时间段。此句型容易与重申句型殽杂，试比力：
It was two years before he came back from abroad.(过了两年他才从外洋归来回头。)
It was two years later before he came back from abroad.(两年之后他才从外洋归来回头。)
第一句为状语从句，it 表现时间；第二句为重申句型，重申的是时间状语two years later。
3.该句型的引申为“还没来得及……就……”。请看例句
Someone called me up in the middle of the night, but they hung up before I could answer the phone.(半夜里有人打电话，但是我还没来得及回复他就挂了。)
4.该句型另有一个引申涵义为“……之后才……”，多与持续性时间、举措连用。比如：
Several weeks had gone by before I realized the painting was missing.(几周之后，我才熟悉到那幅画丢了。
二、“not…before”句型偶尔可换成“hardly…when…”或“scarcely…when”句型
比如：
They had not talked for half an hour before she entered. =They had hardly/scarcely talked for half an hour when she entered.
三、“not…before”句型同“not…until”句型意思相反，表现“直到……才，在……从前不……”(即在否定句中，until 和 till 可以和非一连性动词连用，这时 until 和 before 近义)
比力例句：
She stood there till/until he had passed out of sight.(不必before)他不休站在那儿，直到他散失。
The moon will not come out until/before the clouds disappear.云散了玉轮才会出来。
One doesn’t know the value of health before/until he loses it.人们丢失了康健后才晓得康健的宝贵。
四、Before, after与behind
before表时间、排序或分列时与after相对；表地点时与behind相对。比如：
I saw the headmaster the day before yesterday.我前天见到了校长(时间)。
Autumn comes before winter.秋去冬来(排序)。
A comes before B in the alphabet.在字母表里A排在B之前(分列)。
五、before long与long before
before long是“很快，不久(今后)”的意思，即是soon，可以用于种种时态，比如：
Before long he got married.不久他就完婚了。
而long before是“好久从前”的意思，即是long long ago，寻常用在已往时或已往完成时的句子里。比如：
I saw the film long before.我好久从前就看过这部影戏。
别的，before在不同的语境中会产生不同的涵义，学生、教员在学习、教学中要仔细研讨，加以辨别。比如：
Linda has grown into a beautiful girl before she knows it.不知不觉Linda已长成一个亭亭玉立的少女。
Do it before you forget.赶早入手，以免忘记。
Be a pupil before you become a teacher.先做学生再做教师。
Lucy desired no one for her husband before Charles Darnay.
Lucy非Charles Darnay不嫁。
I’ll shoot myself before I apologize to him.我宁死也不向他致歉。
The ship sailed before the wind.汽船顺风而行。
