I'm a complete beginner in philosophy and have been reading Phaedo for the first time. While reading about Forms in this, I understood that a thing can share in tallness and shortness at the same time, i.e can participate in the opposite forms. Again there is another thing (like fire) that shares in a Form and doesn't admit its opposite. My question is, doesn't this sound like a contradiction? If it is not, then how can we differentiate between these two things? One thing I can come up with is, a thing can only share in opposite Forms when compared with at least two other things. Then again, why are we saying that a thing sharing in a Form will not admit it's opposite one? It seems to me that these arguments are opposing each other. Sorry if the question seems a bit naive(like I said, I'm a complete novice), but this has been troubling me nonetheless. Would really appreciate all of your help.
This part of the Phaedo is teasing out a philosophical nuance, namely the distinction between (what I would call) metrics and objects. You can see Socrates setting that up around 103b, where he says:
You have bravely reminded us, but you do not understand the difference between what is said now and what was said then, which was that an opposite thing came from an opposite thing; now we say that the opposite itself could never become opposite to itself, neither that in us nor that in nature. Then, my friend, we were talking of things that have opposite qualities and naming these after them, but now we say that these opposites themselves, from the presence of which in them things get their name, never can tolerate the coming to be from one another.
In more accessible terms, this is the difference between saying "is X" and saying "is Xer than". The example given later using odd numbers is instructive. Clearly 'Oddness' is constructed in opposition to 'Evenness'. That's the first case, in which the concept 'Odd Number' has no meaning except as measured against the concept 'Even Number'. But no specific odd number is defined in opposition to any specific even number: 3 is not the opposite of 2 or 8, 5 is not the opposite of 4 or 6. Likewise one person might be consider tall when measured against one person (i.e., 'taller') and short when measured against another person (i.e., 'shorter'), but no one can be 'tall' and 'short' at the same time.
Put in more modern terms, we can think of a metric as something which shows difference along a single dimension, and an object-class which shows uniqueness by negating all other things. Thus 'hotter' is a metric along the single dimension of temperature: fire is hotter than snow. But 'fire' is opposed only by things that are 'not-fire'; campfires, forge-fires, and wildfires are a unique category of objects that share common characteristics lacked by other objects. Snow is not the direct opposite of fire, though snow is surely included in the world of things that are not-fire.
This of course leads to some confusions. 'Hot' is an object; 'hotter' is a metric. We need (in Socrates' sense) to have some access to some idealized object 'hot' in order to apply the metric 'hotter', but the two modes need to be kept conceptually separate from each other.
Plato is filled with contradictions, particularly in the Socratic dialogues, and they are generally intentional. The goal is to shock your mind out of ordinary ways of thinking. In this case, Socrates is drawing a contrast between the objects of the ordinary world, which can seemingly partake in opposite qualities, and the greater reality of the conceptual world, in which things are only one thing.
The typical pattern of the Socratic dialogues is to demonstrate the ways in which the ordinary world, and the ordinary ways of seeing the world, don't make sense, and lead to inextricable contradictions. This, in turn, is meant to force you to turn towards the Ideal world.
It's a fairly mainstream opinion that this is a hybrid of the practices of the historical Socrates, the star of the dialogs, with the developed metaphysical theories of Plato, their author. The intensive questioning, leading to the interlocutor facing up to flaws and paradoxes in their logic and beliefs, is authentically Socratic, whereas the mystical realm of the Forms, and the metaphysical cosmos surrounding it is Platonic.