How does the negative energy solution to the Dirac equation predict the antielectron? Please, can someone explain how the negative energy solution can be used to predict the existence of the antielectron?
 A: It was  not a prediction, it was a conjecture, in what was Dirac's hole theory:

The negative E solutions to the equation are problematic, for it was assumed that the particle has a positive energy. Mathematically speaking, however, there seems to be no reason for us to reject the negative-energy solutions.

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any electron placed in a positive-energy eigenstate would decay into negative-energy eigenstates of successively lower energy. Real electrons obviously do not behave in this way, or they would disappear by emitting energy in the form of photons. 
To cope with this problem, Dirac introduced the hypothesis, known as hole theory, that the vacuum is the many-body quantum state in which all the negative-energy electron eigenstates are occupied. This description of the vacuum as a "sea" of electrons is called the Dirac sea. Since the Pauli exclusion principle forbids electrons from occupying the same state, any additional electron would be forced to occupy a positive-energy eigenstate, and positive-energy electrons would be forbidden from decaying into negative-energy eigenstates. 

It was a theory that was superseded by quantum electrodynamics and the problems of the theory were not resolved, although 

In certain applications of condensed matter physics, however, the underlying concepts of "hole theory" are valid. The sea of conduction electrons in an electrical conductor, called a Fermi sea, contains electrons with energies up to the chemical potential of the system. An unfilled state in the Fermi sea behaves like a positively charged electron, though it is referred to as a "hole" rather than a "positron". The negative charge of the Fermi sea is balanced by the positively charged ionic lattice of the material. 

One should read the link and links therein for a clear picture.
A: If you have a complex field, you can write it as a sum of two real fields. Now if you construct the observables for both fields, you'll find that are all identical except the charge operator, that gives a negative charge to one field and positive for the other. Then you have identical particles except for the charge. You can check Quantum Field Theory 2010 from Franz Mandl chapter 2.
