I'll try to do my best and shed some light on this, but I also find the dialogue form frustrating. I forgot which book had Socrate's arguments for the immortality of the soul, so I'm going to include that here to show how Plato's ideas fit together. This is one of Plato's arguments for the reality of the forms and perhaps the immortality of the soul. This is an argument from previous knowledge of the forms OR for existence of the forms. I've taken this apart, so you should be able to place it.
Plato's Phaedo wrote:
‘Well now, with regard to the instances in the logs, and in general, the equals we mentioned just now, are we affected in some way as this: do they seem to us to be equal in the same way as what it is itself? Do they fall short of it at all in being like the equal, or not?’
Plato is discussing the difference between something being exactly similar to something (ie identical twins or two identical cups, are exactly similar in structure but are differentiated by their spatial location) and something that is the same as itself. Say, I have two identical apples in two different spatial locations, apple A and apple B. Is apple A apple B? No. Hence they are not entirely equal.
"We know no two physical things to be entirely equal"
Plato's Phaedo wrote:
‘Then whenever anyone, on seeing a thing, thinks to himself, “this thing that I now see seeks to be like another of the things that are, but falls short, and cannot be like that object: it is inferior”, do we agree that the man who thinks this must previously have known the object he says it resembles but falls short of?’
‘Now then, have we ourselves been affected in just this way, or not, with regard to the equals, and the equal itself?’
‘Indeed we have.’
‘Then we must previously have known the equal, before that time when we first, on seeing the equals, thought that all of them were striving to be like the equal but fell short of it.’
He is saying that we have a concept of perfect versions of real-life objects, but know them to fall short of the mark of the perfect objects, therefore we must have known what the perfect object was like before. There are no perfect objects on this level of reality, so our souls, being the higher part of ourselves, must have at some point had contact with forms, which would exist in a higher reality than physical bodies.
"There are no equal things, but we know that the concept of equal exists. We must have had contact with it previously to know about it. The perfect equal is the form of equal. Forms exist in a higher plane of existence. Our souls exist in a higher plane of existence. Therefore, our souls must have come in contact with the forms prior to our birth."
I hope this clears things up a bit. You really have to read it in context of the rest of the writings. I personally think Aristotle, Epicurus and Epictetus make a lot more sense than Plato. My own personal ethics are a patchwork of the bits of their ethics that I like and their thought is often a lot more rigorous than that of Plato.
