It is not long before they are so accustomed to swimming 这里的

代词 it 有四种用法：
1、人称代词：用于指代已经提到过的事物,或者无生命物体、性别不详或不知的有生命物体（包括人和动物）.如：
* Please polish the table until it shone.请把桌子擦出光亮.(指代table)
* We couldn't find out who it was.我们查不出是谁.（指代不知的人）
* Who is it outside?外面是谁呀?（指代性别不详的人）
2、非人称代词：用于指代时间、天气、距离、情况以及各种度量衡.如：
* It is 5 o’clock.(指代时间)
* It is fine today.(指代天气)
* It is not far from here.(指代距离)
3、先行代词：用作形式主语或宾语,指代后置主语、不定式、动名词或从句.如：
* It is easy to do it.(指代不定式to do it)
* It is helpful going with the times.与时俱进是有益的.(指代动名词going with the times)
* It is important that we get there in time.我们及时到达那里是重要的.(指代动从句that we get there in time)
4、强调代词：用在强调句型“it is + 强调部分 + that …”如：
* It is him that we will pay a visit to.(强调宾语him)
* It was yesterday we paid a visit to him.(强调时间状语yesterday)
显而易见,It is not long before they are so accustomed to swimming（过了不久他们就习惯了游泳）中的 it 是指代时间的非人称代词.直译就是“时间没过多久,他们就习惯了游泳”.其结构是——
主语：It （指代时间）
系动词：is
表语：not long
时间状语从句：before they are so accustomed to swimming
再问： 原来的句子是不是They are so accustomed to swimming is not long before。他们是不久前习惯了游泳。翻译的对不
再答： 那样的话就成了强调句，意思变成了一个病句“正是他们和习惯游泳这件事才是不久以前”。 事实上，它的结构是—— 主句：It is not long ——时间还没有多久 时间状语从句：before they are so accustomed to swimming ——在他们很适应游泳的这段时间以前。