Adding a bit more to Rob's answer, I am going to give you the English equivalent of both the phrases if it helps you: => mae ni > (at) before [In natural English you wouldn't need to say 'at', just 'before' is fine. But, as Rob mentioned, the 'ni' particle specifies the moment something is taking place or happening.] => mae kara > since/from before. (Example: Watashi wa kono koto wo zutto 'MAE' 'KARA' shitteimashita => i knew about this 'FROM' a long time 'AGO'.) Hope my answer was also helpful to you! \(^_^)/

From what I understand. "Mae ni" is used when you want to indicate a specific time period in the past, and what you are talking about is no longer happening. "Mae Kara" is used when you are referring to something that may have started in the past but is still currently happening today and will continue into the future.