but there is another thing you must remember it.为什么不加it?

这是个定语从句,省略了that .这样看吧,通俗点：
but there is another thing.还有另外一件事
you must remember it.你必须要记住它.
它 （it ）指代的就是这件事.
两句合并,就要省略一个,否则就重复了啊