一、It be some time +before…
此句型关键有四种含义，分别阐述如下：
1.该句中主句为将来时态，before从句为一般现在时态；主句为一般过去时，从句通常也为过去的时态。
2.例如：
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，过不了多久他又返回家乡。)
该句型中的时间长为long, a long time, years, months, weeks, days, hours, minutes等时间段。此句型容易与强调句型混淆，试比较：
It was two years before he came back from abroad.(过了两年他才从国外回来。)
It was two years later before he came back from abroad.(两年之后他才从国外回来。)
第一句为状语从句，it 表示时间；第二句为强调句型，强调的是时间状语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答他就挂了。)
4.该句型还有一个引申含义为“……之后才……”，多与持续性时间、动作连用。例如：
Several weeks had gone by before I realized the painting was missing.(几周之后，我才意识到那幅画丢了。
二、“not…before”句型有时可换成“hardly…when…”或“scarcely…when”句型
例如：
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”句型意思相同，表示“直到……才，在……以前不……”(即在否定句中，until 和 till 可以和非延续性动词连用，这时 until 和 before 近义)
比较例句：
She stood there till/until he had passed out of sight.(不用before)他一直站在那儿，直到他消失。
The moon will not come out until/before the clouds disappear.云散了月亮才会出来。
One doesn’t know the value of health before/until he loses it.人们失去了健康后才知道健康的珍贵。
四、Before, after与behind
before表时间、顺序或排列时与after相对；表位置时与behind相对。例如：
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(顺序)。
A comes before B in the alphabet.在字母表里A排在B之前(排列)。
五、before long与long before
before long是“很快，不久(以后)”的意思，等于soon，可以用于各种时态，例如：
Before long he got married.不久他就结婚了。
而long before是“很久以前”的意思，等于long long ago，一般用在过去时或过去完成时的句子里。例如：
I saw the film long before.我很久以前就看过这部电影。
另外，before在不同的语境中会产生不同的含义，学生、教师在学习、教学中要认真研究，加以甄别。例如：
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.趁早动手，免得忘记。
Be a pupil before you become a teacher.先做学生再做老师。
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.我宁死也不向他道歉。
The ship sailed before the wind.轮船顺风而行。
