首先我们来看下It wasn't long before和It won't be long before的大致意思：
It wasn't long before：词性为短语，It wasn't long before是过去完成时的形式，由it（主语）+was not（否定形式）+long（形容词）+before（介词）构成。
It won't be long before：词性为短语，It won't be long before是将来完成时的形式，由it（主语）+will not（否定形式）+be（动词be的不定式形式）+long（形容词）+before（介词）构成。
通过下面的表格我们了解下It wasn't long before和It won't be long before的含义、发音和用法
接下来让我们看下It wasn't long before和It won't be long before的用法区别：
1.时态：一个表示过去的时态，一个表示将来的时态
例子：
-It wasn't long before they reached the top of the mountain.
(It wasn't long before他们达到了山顶。)
-It won't be long before we achieve our goal.
(It won't be long before我们实现我们的目标。)
2.否定形式：一个用not作为否定形式，一个用won't作为否定形式
例子：
-It wasn't long before they realized that they made a mistake.
(It wasn't long before他们意识到自己犯了个错误。)
-It won't be long before we understand the answer to this question.
(It won't be long before我们明白这个问题的答案。)
3.时间概念：一个表示在之前的时间很短，一个表示在之后的时间很短
例子：
-It wasn't long before they walked to the beach.
(It wasn't long before他们走到了海滩。)
-It won't be long before we leave this city.
(It won't be long before我们离开这个城市。)
4.时态形式：一个是过去完成时的形式，一个是将来完成时的形式
例子：
-It wasn't long before they found the lost keys.
(It wasn't long before他们找到了丢失的钥匙。)
-It won't be long before the weather gets warmer.
(It won't be long before天会变暖和。)