What would the meaning of '之前知道' be? And '之前知道了'?
There seems to be a cultural difference in usage here. ‘之前知道’ means you learned this before and still know it. 'I knew' often means 'I've forgotten', unless qualified like below.

A：That girl married that boy.
A:这个女孩和那个男孩结婚了。

B could answer:
B：I know.
B:我知道的。
    or
B：I already know/knew that.
B：我之前就知道了。 

A：Who did that girl marry?
A:她和谁结婚了？
B：I knew, but I can't remember now.
B:我知道是谁，但是我忘了。