I think to understand modes you have to first understand the intervals of a major scale.

Basically Dorian has a flat 3 and a flat 7 and is a minor mode and Lydian has a sharp 4 and is a major mode. But all of this will not make a lot of sense unless you understand intervals.

I suggest you have a look at some of Christopher Schlegels lessons. They have certainly helped me out in my understanding of modes.