If it is true that...

The essence of a simple substance is form alone

and

The essence or quiddity can be understood without understanding anything about its existence

Then it must be true that...

Existence is something other than the essence or quiddity, unless there is something whose quiddity is its very own existence

If it is true that...

The essence or quiddity can be understood without understanding anything about its existence

and

Existence is something other than the essence or quiddity, unless there is something whose quiddity is its very own existence

Then it must be true that...

In every other thing, the thing's existence is one thing, and its essence or quiddity or nature or form is another