Plato in his dialogue The Phaedo says that whereas sticks and stones are both equal and unequal, (so maybe what that means is that each stick is going to be equal to some other sticks and unequal to some other sticks, so equal to the stick on the left maybe but shorter than the stick on its right) the form of equal is going to be just equal, and it won't partake of inequality at all. And it will be the cause of equality in things that are equal, for example, equal sticks and stones.

The forms in intellect are some kind of blueprint or model on which the physical world is based, and something needs to come along and shape the matter in accordance with that blueprint. What does that is soul.

Unlike later Neo-Platonists, Plotinus says that our souls are always connected to the universal intellect and that we never really fall away.

We do have intellects and Plotinus controversially thought that even though we might not be aware of it, our souls are always connected to the intellect. They never fully descend as he would put it.

The soul must be distinct from intellect because even at its best, what the soul does when it's thinking, is it thinks linguistically, it thinks in a temporarily extended way, so it for example, might go through the steps of an argument chain, as if you were going through a syllogism and seeing that something followed from the premises, whereas intellect simply grasps the forms.

When you're seeking after bodily pleasure of food and drink for example, this is going to prevent you from doing what you should be doing which is contemplating.

This is in a way the most important thing about soul is that it's a kind of principle which mediates between the universal intellect and the material world.