Is it possible to teach a parrot the entire English language and have him be able to speak and understand it almost perfectly?

It is not possible. Parrots, even the most intelligent, must a learn a foreign language not even developed by the same race. Without any possible way to communicate with them in their own language, certain things wold not be explainable.

To make it comparable, imagine yourself as a five year-old trying to learn to talk with dogs. No one understands you, and you can only tell what the barks mean by the body language of the dogs themselves.

With African greys being the most intelligent of parrots, they have the most stunnig accomplishments. Some have learned over 5000 Words and use them in the correct context.