Puzzle about the pronunciation of the word 'THE'

The grammer books say 'the' has 2 kinds of pronunciation.I always have a puzzle,when I read a sentence from to right,for example,
The apple is red.
I first see the word 'The',but I can't affirm its pronunciation.I have to look at the second word 'apple',then I will look at 'The' again.,and will

know how to pronunciate 'The'.(Because a['apple'] is a vowel )
I feel the pronunciation of 'the' is so complicated,so I want to know native speaker how to affirm 'the' pronunciation at the different

situations(for instance,the apple,the pen)?