It is not long before they are so accustomed to swimming that they can pick up weights from the floor of the pool.

为什么这个before前后时态一致？有什么规律吗?

   【答】It is not long before ...的意思是“不久就 ...”，如果It 后面用 was,则 before 从句用过去时；如果It 后面用的 won't be,则 before 从句用一般现在时。例如：

    It was not long before the scandal reared its ugly head. 不久这个丑闻就露出了丑恶的面目。With the leaves changing colors, it won't be long before winter is here. 树叶在变颜色了，不久就会到冬天了。（摘自《外研社英汉多功能词典》p.182）

    在网友的句子中，It is not 应该改为 It won't [= will not] be，在 before they are so accustomed to swimming that they can pick up weights from the floor of the pool 这个时间状语从句中，含有一个 so ... that 结构。