Though I'm going to leave the discussion of whether our approach to relations is better than the Greeks for another day, I would like to say some things about how we handle it in logic.
There are different ways that contemporary symbolic logic tends to handle relations like the kinds Parmenides was discussing in the previous post. I'll walk through just one. Let's say that we wanted to express a likeness relation such as "All crows are black." It would look like this:
∀
x(C
x⊃B
x)
That is read, "For every x, if x is a crow then x is black."
Notice that this relationship does not include any actual crows, only a variable x. It is a statement that describes every object in the universe, most of which will fail to be crows. Those things that satisfy the crow condition will, if the statement is true, also satisfy the black condition.
It is possible to talk about an actual crow. Let's say object a is a crow. (Variables are from the end of the alphabet, whereas letters taken from the beginning of the alphabet are constants. That is to say that x or y could be anything, but a is a particular something.) Now you can test the proposition, because if Ca is true and Ba is false, then the proposition is false. It only takes one counterexample to falsify a universally quantified statement like the one above (the upside-down A is the universal quantifier).
So one way we can talk about relations between objects is to use the capital letter to indicate a class of things. All of those things are automatically related to each other by being members of the class.
That doesn't actually solve the problem Parmenides is raising, though, because you still need two things at work to express the relation. Let us say that a is not a crow, but the One or really any Form that is a unitary idea. Being a pure unity, it is just a and not Fa. A genuinely pure unity cannot admit of likeness in this way either.
It also, it turns out, can't admit of unlikeness on this model because you'd have to say that a was not a crow, and that requires three concepts working together: the constant, the class, and the negative operator. (¬ Ca). The pure unity does not admit of either the ¬ or the C, because if it did, it would no longer be just one idea but multiple ideas.
It actually seems like the Form of a Crow would be C, though, not a. After all, it is x's participation in C that makes it a crow. Now you might say that the One is "O," and any x might participate in it without changing it. So you could have Oa and Ob, where a and b each participate in the One without being the One. This addresses Parmenides' concerns somewhat, because whether any x does or doesn't participate in O, O remains singular and unchanged by the participation or lack thereof.
Viewed that way, contemporary symbolic logic depends upon Platonic forms. So too does mathematical logic, and therefore math itself.