In spoken language, people pronounce the definite article “the” in two different ways:
stressed: /ði/
The man enters the bar and looks around. Then he sees her. The woman looks beautiful. When she notices him, she looks into the eyes of the man. During the afternoon he had felt very nervous. Now he thinks that the evening will turn out great. He walks towards the lady , gives her a kiss and hands her the orange flower.
You pronounce the article “the” as /ði/ in front of words that start with a vowel-sound: the apple, the evening, the eye, the example, the oak, the ears, the arm, the idea, the egg.
You pronounce the article “the” as /ðə/ in front of words that start with a consonant-sound: the car, the cat, the sword, the man, the girl, the table, the kitchen, the book, the door.
Note: the indefinite articles “a” or “an” follow the same rules.
Fonte: http://www.learning-english-online.net/pronunciation/the-definite-article-the/