It is not long before they are so accustomed to swimming 这里的it is为什么在前面,是什么用法

it是形式主语,真正的主语是后面的从句they are so accustomed to swimming,其实本来的句子是They are so accustomed to swimming is not long before.因为这个主语太长了,为了避免头重脚轻现象,就把真正的主语放在后...it是不是代指的they are so accustomed to swimming对的，就是指代这个从句。