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