For a bird to learn to talk it must be exposed to human speech. While a very small number of very talented birds can learn to talk from being exposed to normal conversation not directed at the bird, most need direct and numerous repetitions of the phrase to be learned. In most cases this direct training involves someone standing at the cage repeating the same phrase over and over. If you don't have their attention they wont learn anything similar to small children.
Tip: Birds learn by repetition. I recommend using words in context so your bird will have an association for the word being taught. Speak to your bird as you would another person, and you will be amazed how much bird can learn.
Bird Info: Birds do not have a voice box and vocal chords. They reproduce sound through the syrinx. The variations in sounds are finely controlled with several small muscles. Some birds can even whistle or sing two songs simultaneously.